I hope you all liked the round. Please share your feedback in the comments section.
1682A — Palindromic Indices
Read the statement carefully!! The given string is a palindrome.
Let's remove some index $$$i$$$ from the first half of $$$s$$$ and check whether the resulting string is a palindrome or not, the other half has the same approach. The prefix of length $$$i-1$$$ already matches with the suffix of the same length because the initial string was a palindrome, so we just need to check if $$$t = s[i + 1 \ldots n - i + 1]$$$ is a palindrome.
For $$$t$$$ to be a palindrome, $$$s_{n - i + 1}$$$ should be equal to $$$s_{i + 1}$$$ which was initially equal to $$$s_{n - i}$$$, again which should be equal to $$$s_{i + 2}$$$ and this goes on. Here we can see that $$$s_i = s_{i + 1} \ldots = s_{n - i + 1}$$$. So the answer is simply equal to the number of contiguous same characters in the center of the string which can be calculated in $$$\mathcal{O(n)}$$$.
#include<bits/stdc++.h>
using namespace std ;
#define ll long long
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(a) (ll)a.size()
#define F first
#define S second
#define INF 2000000000000000000
#define popcount(x) __builtin_popcountll(x)
#define pll pair<ll,ll>
#define pii pair<int,int>
#define ld long double
template<typename T, typename U> static inline void amin(T &x, U y){ if(y < x) x = y; }
template<typename T, typename U> static inline void amax(T &x, U y){ if(x < y) x = y; }
#ifdef LOCAL
#define debug(...) debug_out(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) 2401
#endif
int _runtimeTerror_()
{
int n;
cin >> n;
string s;
cin >> s;
int cnt = 0;
for(int i=(n-1)/2;i>=0;--i) {
if(s[i] == s[(n - 1) / 2]) {
++cnt;
}
else {
break;
}
}
cout << 2 * cnt - (n & 1) << "\n";
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef runSieve
sieve();
#endif
#ifdef NCR
initncr();
#endif
int TESTS = 1;
cin >> TESTS;
while(TESTS--) {
_runtimeTerror_();
}
return 0;
}
1682B — AND Sorting
You must have to make at least one swap on the elements which are not at their correct positions initially. So $$$X$$$ must be a submask of all elements which are not at their correct positions.
What is the maximum possible value of $$$X$$$ from Hint $$$1$$$? It is the bitwise AND of all elements which are not at their correct positions. It turns out that this value is achievable too.
We always have to make at least one swap for the elements which are not at their correct positions. Hence an upper bound of answer would be the bitwise AND of those elements. Let the value be $$$X$$$. It turns out that the given permutation is $$$X$$$-sortable.
Proof:
First, notice that $$$X$$$ would always be present in $$$p$$$. Let $$$pos_x$$$ be the position of $$$X$$$ in $$$p$$$ initially. Let's say at some point we want to swap two values $$$p_i$$$ and $$$p_j$$$, then $$$p_i$$$ and $$$p_j$$$ would always be a supermask of $$$X$$$ i.e. $$$p_i$$$ & $$$X = X$$$ and $$$p_j$$$ & $$$X = X$$$. We can make the following moves to swap $$$p_i$$$ and $$$p_j$$$ without disturbing any other element.
- Swap values at indices $$$i$$$ and $$$pos_x$$$.
- Swap values at indices $$$i$$$ and $$$j$$$.
- Swap values at indices $$$j$$$ and $$$pos_x$$$.
It can be seen that in every swap the bitwise AND of two values which we are swapping is always $$$X$$$. Hence we can swap any two values which were not at their correct positions, therefore we can sort the permutation $$$p$$$.
Overall Complexity: $$$\mathcal{O(n)}$$$.
#include<bits/stdc++.h>
using namespace std ;
#define ll long long
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(a) (ll)a.size()
#define F first
#define S second
#define INF 2000000000000000000
#define popcount(x) __builtin_popcountll(x)
#define pll pair<ll,ll>
#define pii pair<int,int>
#define ld long double
template<typename T, typename U> static inline void amin(T &x, U y){ if(y < x) x = y; }
template<typename T, typename U> static inline void amax(T &x, U y){ if(x < y) x = y; }
#ifdef LOCAL
#define debug(...) debug_out(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) 2401
#endif
int _runtimeTerror_()
{
int n;
cin >> n;
int ans = (1 << 30) - 1;
for(int i=0;i<n;++i) {
int x;
cin >> x;
if(x != i) {
ans &= x;
}
}
cout << ans << "\n";
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef runSieve
sieve();
#endif
#ifdef NCR
initncr();
#endif
int TESTS = 1;
cin >> TESTS;
while(TESTS--) {
_runtimeTerror_();
}
return 0;
}
1682C — LIS or Reverse LIS?
Let $$$\text{LDS}(a)$$$ be the longest strictly decreasing subsequence of $$$a$$$, then $$$\text{LIS}(a')$$$ = $$$\text{LDS}(a)$$$.
There can be at most one index common between $$$\text{LIS}(a)$$$ and $$$\text{LDS}(a)$$$.
Let's make a small observation:
- There can be at most one index common to both $$$\text{LIS}(a)$$$ and $$$\text{LDS}(a)$$$.
If some element $$$x$$$ occurs $$$\geq 2$$$ times, then one of its occurrences can be included in $$$\text{LIS}(a)$$$ and another one in $$$\text{LDS}(a)$$$, and all the remaining occurrences are of no use because none of them can contain 2 equal elements.
If some element $$$x$$$ is a singleton i.e. the frequency of $$$x$$$ in $$$a$$$ is $$$1$$$, then it can have $$$3$$$ positions
- In $$$\text{LIS}(a)$$$ only.
- In $$$\text{LDS}(a)$$$ only.
- The only common element of $$$\text{LIS}(a)$$$ and $$$\text{LDS}(a)$$$.
It can be seen that it is always optimal to choose some singleton as the only common element (if available) because those with frequency $$$\geq 2$$$ can easily contribute $$$1$$$ to both $$$\text{LIS}(a)$$$ and $$$\text{LDS}(a)$$$ easily.
Let $$$t$$$ be the number of elements having frequency $$$\geq 2$$$ and $$$s$$$ be the number of singletons in $$$a$$$. The singletons should be divided equally among $$$\text{LIS}(a)$$$ and $$$\text{LDS}(a)$$$ with one of them given to both, if available.
Hence, the answer is $$$t + \lceil \frac{s}{2} \rceil$$$.
The values $$$s$$$ and $$$t$$$ can be found using some data structure like $$$\text{std:map}$$$ in C++ in $$$\mathcal{O}(n\log(n))$$$.
#include<bits/stdc++.h>
using namespace std ;
#define ll long long
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(a) (ll)a.size()
#define F first
#define S second
#define INF 2000000000000000000
#define popcount(x) __builtin_popcountll(x)
#define pll pair<ll,ll>
#define pii pair<int,int>
#define ld long double
template<typename T, typename U> static inline void amin(T &x, U y){ if(y < x) x = y; }
template<typename T, typename U> static inline void amax(T &x, U y){ if(x < y) x = y; }
#ifdef LOCAL
#define debug(...) debug_out(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) 2401
#endif
int _runtimeTerror_()
{
int n;
cin >> n;
map<int, int> mp;
for(int i=1;i<=n;++i) {
int x;
cin >> x;
++mp[x];
}
int single = 0, doble = 0;
for(auto &[i, j]:mp) {
single += j == 1;
doble += j > 1;
}
cout << doble + (single + 1) / 2 << "\n";
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef runSieve
sieve();
#endif
#ifdef NCR
initncr();
#endif
int TESTS = 1;
cin >> TESTS;
while(TESTS--) {
_runtimeTerror_();
}
return 0;
}
1682D — Circular Spanning Tree
What are the mandatory conditions on string $$$s$$$ for a tree to be possible?
If there are no odd degree vertices or the count of odd degree vertices is odd, then it is impossible to construct any tree. It turns out that these conditions are sufficient too.
Let's check some cases when it is not possible to construct the answer-
- When all vertices have an even degree, then there is no way to generate a tree because every tree contains at least $$$2$$$ leaves.
- When there are an odd number of vertices with odd degrees, then there is no tree possible because the sum of degrees must be even.
It turns out that it is always possible to construct a tree if none of the above is true.
The following construction works -
Select some vertex $$$i$$$ such that the previous vertex of $$$i$$$ (assumed cyclically) has an odd degree i.e. $$$s_{i - 1} = 1$$$. Clearly, such a vertex always exists.
Now left rotate $$$s$$$, $$$i - 1$$$ times such that the selected vertex is now at index $$$1$$$. Note that after the rotation $$$s_n$$$ will become $$$1$$$. Now we can see that $$$s[2\ldots n]$$$ can be divided into several segments such that each segment ends with some vertex having an odd degree. And each segment should contain exactly one vertex with an odd degree. So $$$s[2 \ldots n] = [0\ldots 1][0\ldots 1] \ldots [0\ldots 1]$$$ where $$$0$$$ may appear $$$0$$$ times. Connect vertex $$$1$$$ to the starting vertex of each segment and connect adjacent vertices inside each segment. It can be clearly seen that edges will never intersect internally. The only thing we need to verify is the degree constraints.
Proof:
- The degree condition is valid for each segment, as each vertex with an even degree is connected with $$$2$$$ other vertices and the last vertex with an odd degree will be connected to only one vertex i.e it's previous one or vertex $$$1$$$ if it was only on its segment.
- Let $$$cnt_1$$$ be the number of vertices with odd degree. If $$$s_1 = 1$$$, then there will be $$$cnt_1 - 1$$$ segments which is an odd number, hence vertex $$$1$$$ will be connected to odd number of vertices. If $$$s_1 = 0$$$, then there will be $$$cnt_1$$$ segments which is an even number, hence vertex $$$1$$$ will be connected to even number of vertices.
Note that we renumbered the vertices during rotation which should be handled in implementation.
The intuition for the above approach comes from the case when all $$$s_i$$$ are $$$1$$$ in which we create a star network.
Overall complexity: $$$\mathcal{O(n)}$$$.
#include<bits/stdc++.h>
using namespace std;
#define all(v) v.begin(),v.end()
#define endl "\n"
int solve(){
int n; cin >> n;
string s; cin >> s;
auto cnt = count(all(s),'1');
if(cnt == 0 or cnt & 1){
cout << "NO" << endl;
return 0;
}
auto inc = [&](int j){
return (j + 1)%n;
};
cout << "YES" << endl;
for(int p = 1; p < n; p++){
if(s[p - 1] == '1'){
auto i = inc(p);
while(i != p){
int j = i;
int prev = p;
while(j != p){
cout << prev + 1 << " " << j + 1 << endl;
prev = j;
j = inc(j);
if(s[prev] == '1')break;
}
i = j;
}
return 0;
}
}
return 0;
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
#ifdef SIEVE
sieve();
#endif
#ifdef NCR
init();
#endif
int t=1;cin>>t;
while(t--){
solve();
}
return 0;
}
1682E — Unordered Swaps
One way of solving permutation problems is to look at permutation cycles. Let's decompose our permutation into cycles, then it's easy to see that each cycle can be solved independently because we have to sort the permutation in a minimum number of moves which isn't possible if two cycles are merged at any instant.
Let's look at one cycle only, whose vertices are numbered from $$$1$$$ to $$$n$$$ in the orientation of cycle i.e the cycle is $$$1 \rightarrow 2 \rightarrow \ldots \rightarrow n \rightarrow 1$$$. Also assume that we only have swaps $$$(x, y)$$$ that are relevant to this cycle.
It is known that we can sort a cycle of size $$$n$$$ in $$$n - 1$$$ moves and it is the minimum number of required moves.
Claim 1: The set of swaps if considered as edges $$$(x, y)$$$ form a tree on the $$$n$$$ vertices of the cycle.
Assume that the edges don't form a tree, then there exist at least two disjoint components let's say $$$S$$$ and $$$T$$$. Now we must be able to make swaps inside $$$S$$$ only, to sort elements in $$$S$$$ which needs that the set {$$$i, i \in S$$$} is same as set {$$$p_i, i \in S$$$} which is not possible by property of permutation cycles. Any cycle of permutation, say $$$C$$$, can't be split further into two sets $$$S$$$ and $$$T$$$ such that both of them can be sorted independently among themselves.
So we must use all the $$$n - 1$$$ edges of the tree in some order to get $$$n$$$ cycles each of size $$$1$$$.
Let's consider any element $$$u$$$ having adjacency list as $$$[x_1, x_2, ..., x_k]$$$ in the order they appear on the cycle if we start moving from $$$u$$$ in the orientation of cycle i.e $$$u \rightarrow u + 1 \rightarrow ... \rightarrow n \rightarrow 1 \rightarrow ... \rightarrow u$$$.
Claim 2: We can never make the swap $$$(u, x_j)$$$ before swap $$$(u, x_i)$$$ if $$$j > i$$$.
If we make the swap $$$(u, x_j)$$$ first, then $$$u$$$ and $$$x_i$$$ will go in different cycles for subsequent operations and we will never be able to use edge $$$(u, x_i)$$$ because it will merge the two different cycles which isn't possible because we are constrained to break a cycle into smaller cycles only.
And if are not able to use edge $$$(u, x_i)$$$ then we will never be able to sort the permutation because we had $$$n - 1$$$ edges all of which were to be used and we wasted $$$1$$$ of them.
Using above claim, for every element $$$u$$$ the order of edges is fixed i.e $$$x_1$$$, then $$$x_2$$$, ..., and finally $$$x_k$$$.
Let's build a directed graph on $$$n - 1$$$ vertices (representing the swaps) where for every element $$$u$$$ we add directed edges $$$(u,x_1) \rightarrow (u,x_2)$$$, ..., $$$(u,x_{k-1}) \rightarrow (u,x_k)$$$.
Since it is guaranteed that the answer will exist i.e a valid sequence of moves exist, hence the topological sorting of the above graph must exist, any of which represents a correct sequence of swaps.
Note that whenever we make a swap that is not violating claim $$$2$$$ for any element $$$u$$$, then there will be no cross edge in two smaller cycles that are going to be formed and those smaller cycles can be further solved independently. Also the order of edges i.e $$$[x_1, x_2, ..., x_k]$$$ is not going to change for any element which ensures that the directed graph we built remains correct even if we remove some appropriate edge.
Hence the answer is simply the topological sort of the graph we built.
Overall Complexity: $$$\mathcal{O(nlog(n))}$$$, $$$nlog(n)$$$ for sorting the edges according to cycle's orientation to get the order $$$[x_1, x_2, ..., x_k]$$$ for every vertex.
Claim: The given swaps considered as edges $$$(x, y)$$$ forms a non-intersecting tree on $$$n$$$ vertices on the circle i.e no two edges intersect internally. Motivation for problem D
Let's say edges $$$(a, b)$$$ and $$$(c, d)$$$ intersect internally in the circle.
WLOG, let's suppose we make swap $$$(a, b)$$$ before swap $$$(c, d)$$$, then $$$c$$$ and $$$d$$$ will go in different cycles as in Claim $$$2$$$ above.
What if you were given any tree on $$$n$$$ vertices and asked to solve the problem with "YES/NO"?
- If the given edges intersect internally in the circle then the answer is "NO" otherwise it's always possible to construct a valid sequence of swaps. This is what the validator of E and checker of D do, try this one, and feel free to discuss in the comments section.
Let's make every edge $$$(u, v)$$$ such that $$$u \lt v$$$, clearly the order of $$$u$$$, $$$v$$$ doesn't matter.
Consider each edge as a segment $$$[u, v]$$$, then the edges of the tree intersect internally if and only if any two segments say $$$[l_1, r_1]$$$ and $$$[l_2, r_2]$$$ satisfies any of the below conditions-
$$$l_1\lt l_2 \lt r_1 \lt r_2$$$
$$$l_2 \lt l_1 \lt r_2 \lt r_1$$$
In the original problem, it was mentioned that there is always a correct sequence of swaps so we claimed that topological sorting must exist and indeed any topological sorting suffices. What if we were given a non-intersecting spanning tree? Can we still claim that there exists a correct move at every step?
Claim: Yes, we can
We need to show that there there is always some edge that can be removed without breaking claim $$$2$$$ above which is the only required condition.
Cycles of length $$$\le 2$$$ are trivial.
Let's represent by $$$u_{next}$$$ the first element of the list $$$[x_1, x_2, ..., x_k]$$$ for $$$u$$$ i.e the closest vertex having an edge with $$$u$$$ in cycle's orientation.
Now, let's start an iteration, start moving from $$$1$$$ and jump to $$$v_{next}$$$ every time when you are at vertex $$$v$$$. Continue this process until you again reach $$$1$$$ or cross over $$$1$$$.
Let the sequence obtained be $$$s$$$ i.e $$$s_1 = 1, s_2, ..., s_k$$$ where on moving further from $$$s_k$$$ we cross/reach $$$1$$$. For simplicity assume $$$k \ge 3$$$, $$$k = 2$$$ is trivial.
It can be shown that $$$(s_{k-1}, s_k)$$$ is the required edge.
$$$s_{k_{next}}$$$ lies between $$$s_{k-1}$$$ and $$$s_k$$$. There are three cases other that this:
$$$s_{k_{next}}$$$ lies between $$$s_k$$$ and $$$1$$$, which is not possible because we would have moved further and $$$s_k$$$ would not be the last element of sequence $$$s$$$.
$$$s_{k_{next}}$$$ = $$$1$$$ which is not possible because it will create a cycle and we are given a tree.
$$$s_{k_{next}}$$$ lies between $$$s_j$$$ and $$$s_{j+1}$$$ for $$$j \le k-2$$$, this is also not possible because then the edges $$$(s_k, s_{k_{next}})$$$ and $$$(s_j, s_{j+1})$$$ will intersect and we are given a non-intersecting tree.
$$$s_{k}$$$ is first element of adjacency list of $$$s_{k-1}$$$ by the definition of $$$v_{next}$$$ and $$$s_{k-1}$$$ is the first element of adjacency list of $$$s_{k}$$$ by above 3 points.
Hence it is safe to make the swap $$$(s_{k-1}, s_{k})$$$.
So the topological sort still works.
This might not be the only proof, if you have some other proofs feel free to discuss them in the comments.
Hope you liked the details!!
Any ideas on how to write a generator for this problem?
Randomly partition the permutation into cycles, so generating swaps for a particular cycle is the main issue.
Let's represent the cycle by an array $$$a$$$ of size $$$n$$$ with cycle as $$$a_1 \rightarrow a_2 \rightarrow ... \rightarrow a_n \rightarrow a_1$$$ Now let's start making random swaps say $$$(a_i, a_j)$$$ to break the cycle, then this generates two smaller cycles -
$$$a_1 \rightarrow a_2 \rightarrow ... \rightarrow a_i \rightarrow a_{j+1} \rightarrow ... \rightarrow a_n \rightarrow a_1$$$.
$$$a_{i+1} \rightarrow ... \rightarrow a_j \rightarrow a_{i+1}$$$.
This can be easily done using treaps :) and then we can use recursion to solve them independently.
It's very rare!! Atleast first time for us.
#include<bits/stdc++.h>
using namespace std ;
#define ll long long
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(a) (ll)a.size()
#define F first
#define S second
#define INF 2000000000000000000
#define popcount(x) __builtin_popcountll(x)
#define pll pair<ll,ll>
#define pii pair<int,int>
#define ld long double
template<typename T, typename U> static inline void amin(T &x, U y){ if(y < x) x = y; }
template<typename T, typename U> static inline void amax(T &x, U y){ if(x < y) x = y; }
#ifdef LOCAL
#define debug(...) debug_out(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) 2401
#endif
const int N = 2e5 + 5;
vector<int> t_sort;
int idx[N];
int vs[N];
vector<int> v[N];
bool dfs_sort(int u)
{
vs[u]=2;
for(auto j:v[u])
{
if(vs[j]==2)
return true;
if(vs[j]==0 && dfs_sort(j))
return true;
}
vs[u]=1;
t_sort.push_back(u);
return false;
}
// Returns true if there is a topological sort else returns false
bool top_sort(int n)
{
t_sort.clear();
for(int i=1;i<=n;++i)
vs[i]=0;
for(int i=1;i<=n;++i)
{
if(vs[i]==0)
{
if(dfs_sort(i))
{
t_sort.clear();
return false;
}
}
}
reverse(t_sort.begin(),t_sort.end());
assert(t_sort.size()==n);
for(int i=0;i<n;++i)
idx[t_sort[i]]=i;
return true;
}
int _runtimeTerror_()
{
int n, k;
cin >> n >> k;
vector<int> p(n+1), a(k), b(k);
for(int i=1;i<=n;++i) {
cin >> p[i];
}
vector<vector<pii>> g(n+1);
for(int i=0;i<k;++i) {
int x, y;
cin >> x >> y;
a[i] = x, b[i] = y;
g[x].push_back({y, i + 1});
g[y].push_back({x, i + 1});
}
vector<int> id(n+1);
vector<int> ans;
auto solve = [&](vector<int> &cyc) {
int n = sz(cyc);
if(n == 1) {
return;
}
for(int i=0;i<n;++i) {
id[cyc[i]] = i;
}
auto dist = [&](int x, int y) {
return (id[y] - id[x] + n) % n;
};
vector<int> good;
for(int i:cyc) {
sort(all(g[i]), [&](pii &a, pii &b) {
return dist(i, a.F) < dist(i, b.F);
});
for(int j=1;j<sz(g[i]);++j) {
v[g[i][j-1].S].push_back(g[i][j].S);
}
}
};
vector<bool> vis(n+1);
for(int i=1;i<=n;++i) {
if(vis[i]) {
continue;
}
vector<int> cycle;
int cur = i;
while(!vis[cur]) {
cycle.push_back(cur);
vis[cur] = 1;
cur = p[cur];
}
solve(cycle);
}
top_sort(k);
for(auto i:t_sort) {
cout << i << " ";
}
cout << "\n";
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef runSieve
sieve();
#endif
#ifdef NCR
initncr();
#endif
int TESTS = 1;
//cin >> TESTS;
while(TESTS--) {
_runtimeTerror_();
}
return 0;
}
1682F — MCMF?
Let us suppose we need to calculate the answer for only one query, say complete array i.e $$$a[1:n]$$$.
The scary flow structure in the problem can be reduced as-
Let's replicate each vertex $$$i$$$, $$$|b_i|$$$ times. Then we can see that there will be an equal number of vertices on the left and right side. Now the problem reduces that we have to match these vertices with minimum cost such that the cost of matching $$$i$$$ and $$$j$$$ is $$$|a_i - a_j|$$$.
There are only 2 type of elements (left side and right side) and the following greedy algorithm to match the elements works.
Algorithm: Sort the type $$$1$$$ and $$$2$$$ elements independently and match them in the sorted order.
Assume that two elements from left $$$l_1 \le l_2$$$ are matched with two elements from right $$$r_1 \le r_2$$$ as $$$[l_1, r_2]$$$ and $$$[l_2, r_1]$$$, then it can be easily shown that matching $$$[l_1, r_1]$$$ and $$$[l_2, r_2]$$$ is always more optimal. The proof is left as an excercise to reader.
Since the array $$$a$$$ is given in sorted order, let's use it!!
Let's assume-
Type $$$1$$$ elements are those having $$$b_i \lt 0$$$.
Type $$$2$$$ elements are those having $$$b_i \gt 0$$$.
Now instead of replicating elements $$$|b_i|$$$ times and sorting them independently, let's iterate on array $$$a$$$ from left to right and add the contribution of each element independently. Say we are at index $$$i$$$, and prefix sum of $$$b_i$$$ so far is $$$psum_i$$$, then the following cases arise-
$$$b_i \gt 0$$$, $$$psum_i \ge 0$$$ — There is no unmatched type $$$1$$$ element on the left, so we just add this element's contribution to the answer i.e $$$-b_i \cdot a_i$$$.
$$$b_i \gt 0$$$, $$$psum_i \lt -b_i$$$ — There are more than $$$b_i$$$ unmatched type $$$1$$$ elements on the left, so we match $$$b_i$$$ of them to $$$a_i$$$, adding a contribution of $$$a_i \cdot b_i$$$ to the answer.
$$$b_i \gt 0$$$, $$$psum_i \lt 0$$$ and $$$psum_i \gt -b_i$$$ — There are less than $$$b_i$$$ unmatched elements ($$$= |psum_i|$$$) on the left, so we match those with equal number of $$$a_i$$$ and remaining are propagated further, adding a contribution of $$$|psum_i| * a_i - (b_i - |psum_i|) * a_i$$$, where the positive term comes from those matching with previous unmatched elements and the negative term comes from those that are going to be left unmatched.
Similar cases are there for $$$b_i \lt 0$$$.
Ok so now we can easily solve the problem for one query in $$$O(n)$$$.
Main idea:
Let's simulate the above algorithm for every suffix and record the obtained answer in $$$ans_i$$$ for $$$i^{th}$$$ suffix. Note that the value $$$ans_i$$$ doesn't denote any answer for some suffix because the sum of $$$b_i$$$ over that suffix might or might not be zero. One important observation here is that-
- Let some subarray $$$a[l:r]$$$ for which sum of $$$b_i$$$ is $$$0$$$, then $$$ans_l - ans_{r+1}$$$ do have a good meaning, it's the answer for that query indeed.
Our answer for $$$a[l:r]$$$ would have been the result of simulation on the subarray, but how does simulation on $$$l^{th}$$$ suffix looks?
It greedily matches the subarray $$$a[l:r]$$$ first because the sum of $$$b_i$$$ is zero, so it will surely pair up all elements in that subarray. Then it moves further on $$$r+1$$$ and continuing the simulation after $$$r+1$$$ is equivalent to starting the simulation from $$$r+1$$$ itself because $$$psum$$$ so far (defined above) would be automatically 0.
Note that $$$ans_l$$$ doesn't have any physical meaning because it will add some junk value if elements after $$$r+1$$$ are not paired up equally but those junk values are exactly same in $$$ans_l$$$ and $$$ans_r$$$ which cancel out, giving the correct answer.
But still, we can't simulate for every suffix, right? It would go $$$O(n^2)$$$ again.
Let's iterate from left to right and for every $$$i$$$ try calculating it's contribution in $$$1^{st}$$$, $$$2^{nd}$$$, ..., $$$(i-1)^{th}$$$ suffixes which is easy because it depends only on $$$psum_i$$$, $$$b_i$$$ (which are constant for a given $$$i$$$) and $$$psum_l$$$ for contribution to $$$l^{th}$$$ suffix. This is pretty standard using $$$2$$$ fenwick trees.
How to calculate $$$ans_i$$$?
Let's solve $$$b_i \gt 0$$$ and $$$b_i \lt 0$$$ independently, say $$$b_i \gt 0$$$ for now. Other case is similar.
Let $$$psum_i = \sum_{j=1}^{i}b_j$$$.
Consider the contribution of index $$$i$$$ to $$$ans_l$$$ for $$$l \lt i$$$, from three cases described above the contribution is different for different $$$l$$$ with different $$$psum_l$$$. We can build a fenwick tree on compressed prefix sums. Case $$$1$$$ and $$$2$$$ above add a constant value to a range of prefix sums that can be maintained in one fenwick tree and Case $$$2$$$ gives some linear function of $$$psum$$$ to be added in a range that can be maintained in other fenwick tree. Add contribution of each $$$i$$$ from $$$1$$$ to $$$n$$$ first, and let's start calculating $$$ans_i$$$.
For $$$i = 1$$$, $$$ans_1$$$ can be obtained by querying at $$$psum_1$$$ in both fenwicks.
Then we remove the contribution of $$$i = 1$$$ from the two fenwick trees (simply the negative of which we added above), because $$$i = 1$$$ won't be contributing to any suffix other than $$$1^{st}$$$ one.
Similarly we move from left to right and calculate $$$ans_i$$$ by querying at $$$psum_i$$$ and then remove the contribution of $$$i^{th}$$$ element.
#include<bits/stdc++.h>
using namespace std ;
#define ll long long
#define pb push_back
#define all(v) v.begin(),v.end()
#define sz(a) (ll)a.size()
#define F first
#define S second
#define INF 2000000000000000000
#define popcount(x) __builtin_popcountll(x)
#define pll pair<ll,ll>
#define pii pair<int,int>
#define ld long double
template<typename T, typename U> static inline void amin(T &x, U y){ if(y<x) x=y; }
template<typename T, typename U> static inline void amax(T &x, U y){ if(x<y) x=y; }
#ifdef LOCAL
#define debug(...) debug_out(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) 2401
#endif
const int MOD=1000000007;
struct Mint {
int val;
Mint(long long v = 0) {
if (v < 0)
v = v % MOD + MOD;
if (v >= MOD)
v %= MOD;
val = v;
}
static int mod_inv(int a, int m = MOD) {
int g = m, r = a, x = 0, y = 1;
while (r != 0) {
int q = g / r;
g %= r; swap(g, r);
x -= q * y; swap(x, y);
}
return x < 0 ? x + m : x;
}
explicit operator int() const {
return val;
}
Mint& operator+=(const Mint &other) {
val += other.val;
if (val >= MOD) val -= MOD;
return *this;
}
Mint& operator-=(const Mint &other) {
val -= other.val;
if (val < 0) val += MOD;
return *this;
}
static unsigned fast_mod(uint64_t x, unsigned m = MOD) {
#if !defined(_WIN32) || defined(_WIN64)
return x % m;
#endif
unsigned x_high = x >> 32, x_low = (unsigned) x;
unsigned quot, rem;
asm("divl %4\n"
: "=a" (quot), "=d" (rem)
: "d" (x_high), "a" (x_low), "r" (m));
return rem;
}
Mint& operator*=(const Mint &other) {
val = fast_mod((uint64_t) val * other.val);
return *this;
}
Mint& operator/=(const Mint &other) {
return *this *= other.inv();
}
friend Mint operator+(const Mint &a, const Mint &b) { return Mint(a) += b; }
friend Mint operator-(const Mint &a, const Mint &b) { return Mint(a) -= b; }
friend Mint operator*(const Mint &a, const Mint &b) { return Mint(a) *= b; }
friend Mint operator/(const Mint &a, const Mint &b) { return Mint(a) /= b; }
Mint& operator++() {
val = val == MOD - 1 ? 0 : val + 1;
return *this;
}
Mint& operator--() {
val = val == 0 ? MOD - 1 : val - 1;
return *this;
}
Mint operator++(int32_t) { Mint before = *this; ++*this; return before; }
Mint operator--(int32_t) { Mint before = *this; --*this; return before; }
Mint operator-() const {
return val == 0 ? 0 : MOD - val;
}
bool operator==(const Mint &other) const { return val == other.val; }
bool operator!=(const Mint &other) const { return val != other.val; }
Mint inv() const {
return mod_inv(val);
}
Mint power(long long p) const {
assert(p >= 0);
Mint a = *this, result = 1;
while (p > 0) {
if (p & 1)
result *= a;
a *= a;
p >>= 1;
}
return result;
}
friend ostream& operator << (ostream &stream, const Mint &m) {
return stream << m.val;
}
friend istream& operator >> (istream &stream, Mint &m) {
return stream>>m.val;
}
};
template<typename T=long long>
struct fenwick {
vector<T> bit;
int n;
fenwick(int x) {
n = x;
bit.resize(x + 1, T(0));
}
void update(int j,T val)
{
for(;j<=n;j+=j&-j)
bit[j] += val;
}
T get(int r)
{
T u = 0;
for(;r;r-=r&-r)
u += bit[r];
return u;
}
T query(int l,int r)
{
return get(r)-get(l-1);
}
// kth element
int getKth(T k) {
int ans = 0;
T cnt = 0;
for(int i=20;i>=0;--i) {
if(ans + (1 << i) <= n && cnt + bit[ans + (1 << i)] < k) {
ans += (1 << i);
cnt += bit[ans];
}
}
if(ans == n) {
return -1;
}
return ans + 1;
}
void insert(int x) {
update(x, 1);
}
void erase(int x) {
update(x, -1);
}
};
int _runtimeTerror_()
{
int n;
int Q;
cin >> n >> Q;
vector<array<int,2>> a(n);
for(int i=0;i<n;++i) {
cin >> a[i][0];
}
for(int i=0;i<n;++i) {
cin >> a[i][1];
}
vector<Mint> val(n, 0);
for(int i=n-1;i>=0;--i) {
if(i < n - 1) {
val[i] = val[i + 1];
}
val[i] += a[i][0] * Mint(abs(a[i][1]));
}
auto solve = [&](vector<array<int,2>> &a) {
ll psum = 0;
vector<ll> psums;
psums.push_back(0);
for(int i=0;i<n;++i) {
psum += a[i][1];
assert(a[i][1] != 0);
psums.push_back(psum);
}
sort(all(psums));
psums.resize(unique(all(psums)) - psums.begin());
psum = 0;
auto get_next = [&](ll x) {
return lower_bound(all(psums), x) - psums.begin() + 1;
};
fenwick<Mint> f1(2*n), f2(2*n), f3(2*n);
for(int i=0;i<n;++i) {
if(a[i][1] > 0) {
f1.update(1, Mint(a[i][1]) * a[i][0]);
f1.update(get_next(psum), -Mint(a[i][1]) * a[i][0]);
f2.update(get_next(psum), a[i][0]);
f2.update(get_next(psum + a[i][1]), -a[i][0]);
f3.update(get_next(psum), Mint(a[i][0]) * Mint(psum + a[i][1]));
f3.update(get_next(psum + a[i][1]), -Mint(a[i][0]) * (psum + a[i][1]));
}
psum += a[i][1];
}
psum = 0;
for(int i=0;i<n;++i) {
val[i] -= 2 * f1.get(get_next(psum));
val[i] -= 2 * (f3.get(get_next(psum)) - psum * f2.get(get_next(psum)));
if(a[i][1] > 0) {
f1.update(1, -Mint(a[i][1]) * a[i][0]);
f1.update(get_next(psum), Mint(a[i][1]) * a[i][0]);
f2.update(get_next(psum), -a[i][0]);
f2.update(get_next(psum + a[i][1]), a[i][0]);
f3.update(get_next(psum), -Mint(a[i][0]) * Mint(psum + a[i][1]));
f3.update(get_next(psum + a[i][1]), Mint(a[i][0]) * (psum + a[i][1]));
}
psum += a[i][1];
}
};
solve(a);
for(int i=0;i<n;++i) {
a[i][1] = -a[i][1];
}
solve(a);
val.push_back(0);
while(Q--) {
int l, r;
cin >> l >> r;
--l, --r;
cout << val[l] - val[r + 1] << "\n";
}
return 0;
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef runSieve
sieve();
#endif
#ifdef NCR
initialize();
#endif
int TESTS = 1;
//cin >> TESTS;
while(TESTS--)
_runtimeTerror_();
return 0;
}
:holyfrick: That was lightining fast .. Thnx for the hinted editorial !! helps a lot in upsolving :)
great problems, fast editorial, quick rating changes, and becoming specialist.
thanks alot :D
Congratulations! I have become specialist too... again)))
Thanks, Congrats on that :)
Missed C by just "+1", took single/2 instead of (single+1)/2. :(
I wish authors who put an anti-hash test in C a very pleasant evening.
Did authors include anti-hash test, or is it somebody's hack?
authors did
How can you be so confident? lol
No, we are not that cruel (At some point, I was also an expert).
oh, sorry, than it must have been a very lucky coincidence for me lol
Click 1 Click 2
Rating change is faster than editorial :D thanks
Is E related to something like toposort(finding indegree = 0)?
Can someone give hint please
make the swaps in permutation into trees, slove the problem in it, and then using toposort.
Can you elloborate on the idea ? I can't get how to construct a tree form permutation Thanks!
If we connect $$$i$$$ and $$$p_i$$$ in a graph, you will see that the graph is some loops. So for one loop with length $$$k$$$, the minimal number of swap must be $$$k-1$$$ and the swap must connect all node in the loop. So you will see that if you connet all swap $$$(a_i,b_i)$$$, it will look like a forest.
Yes, but I have no idea how to solve it on the tree
OK. For an element in position $$$i$$$, we need to move it to position $$$p_i$$$. We will see that we can only use a series of swaps to move $$$p_i$$$, and two node have only one path in a tree. So if the edges in the path from $$$i$$$ to $$$p_i$$$ is represented as $$$x_1,x_2,\dots$$$, they must be used in such an order in the final. So we can make a new graph to describe the order for the swaps, and then use a topo sort can slove it.
But it seems that the number of edges is $$$O(n^2)$$$..?
Oh, I forgot it. A good observation in this problem is that one edge may appear at most twice in all paths. Because A swap edge can only change two position, so it can be proved that if three or more paths through an edge, there must have no solution. So there are at most $$$2n$$$ visit for all the edges. Finally, the time will be $$$O(n)$$$.
got it, thx
It is unbelievable that I send all the solution in the discussion.
lol
I only have an $$$O(n^2)$$$ algorithm. For each loop, choose one node as the starting point, then judge whether the dfs order on the tree is same as the loop's order.
You can see my explanation in the front.
Awesome! Thank you very much
Awesome round. Fast Editorial. Quick Rating Changes. rivalq, CoderAnshu supremacy.
Can anyone tell me what is wrong with my code idea for B https://codeforces.net/contest/1682/my
I just edited one line on your code!
https://ideone.com/58ekJI
Just gonna share my construction for D here (no clue whether it's the same as the editorial):
Red crosses are odd degree vertices. The black lines trace out edges between even degree vertices. Blue lines are edges between odd degree vertices. Also, imagine I drew a circle.
Submission: 158082919 (the code isn't very neat)
What do you do when there are two ones at the end?
If the upper or lower segments of even degree vertices do not exist, it is fine to connect them to the next available point.
You may be thinking that the previous 1 will have even degree, but it will be compensated by a blue edge.
For example, this is what I get when I try to follow the intuition from the picture to construct the tree on string 10001100 (crosses are vertices with odd degrees, circles — vertices with even degrees)
If I follow the intuition from the picture, I end up with a 1 with an even degree.
Well actually the answer is NO for this testcase since the number of 1s is odd.
For question C an input like 1,2,9,8,10,11,11 should have 3 as an answer right but the code will give 4 as the answer. Can someone tell me what am i missing?
Put 1 in the middle.
the round was lucky for me; I just became specialist ;)
Another easy to see proof for A:
Number of element equal to the middle element has the same parity as that of N. So decrease N by 1, the parity of the middle elements should be changed
Here's a pictorial solution to D which is very similar to the one in the editorial.
Assume that the red dots are odd guys and there are lots of even guys on the boundaries. The blue curves denote the edges. The green one is a special case to handle some leftover even guys.
The one in the editorial just picks a pivot adjacent to a red dot so that the green curve looks like a blue one. Observe that the parity of the degree of the pivot is correct irrespective of its choice.
ありがとう
I think C solution is not correct, as it doesn't contemplate all possible cases. The one concretely isn't ok on the pretests 2 is the input num 22:
1 1 2 3 3
Where it says answer is 3. If it was the 1 or the 3 the number that had a single appearance it would be right, as it could be put in the middle of the array, but when is the 2 there is no way to order it that gives result 3.
I made this awful code just to test it and it seems that the max increasing substring is in fact 2
Am I missing something?
The answer is 1 3 2 3 1. Your testing code is just not corrected in finding LIS.
what if the sequence given is : 2 2 3 5 5 7 7
how do you get 4?
what arrangements?
I liked problem F, but I feel like it would've been better to not add the artificial complexity from the bipartite flow graph stuff.
I'd prefer something like "there is a wall of stacked 1 by 1 blocks along the number line. for most of the infinite number line the height is average, but unfortunately there are some positions where there are more or fewer than average blocks in a single column. can you figure out how many steps to the left and right in total the boxes need to be moved? The non-average heights are given in this format with queries..."
I second that, it'd be better this way
A shameful round for me only solved A-C ): :(
Great Round, enjoyed it a lot!!!
in problem B, with the input:
1
8
0 1 2 7 4 5 3 6
it seems like the answer is not correct, if im wrong, pls point out my mistakes.
what is your ans?
Man the solution for D blew my mind :D Great Round!
please add the problem rating in the tags of the problems.
sometimes map is faster than unordered_map
It was not intuitive or provable in the contest that maximum X would be the AND of all misplaced elements of the array. Bad day for me:)
Changing just one line in my problem C code gave AC after the contest :(
For problem B, why the upper bound of the answer is the bitwise AND of all elements which are not at their correct positions.
Because X must be a submask of all such elements and bitwise AND is maximum of those X.
Thanks:)
In question C , on using
unordered_map<int,int>, I am getting TLE whereas usingmap<int,int>accepts the solution. Can anyone please tell why it is so? My both submissions : Accepted Solution using map [problem:C][TLE submission using unordered_map](https://codeforces.net/contest/1682/submission/158110821)unordered map answers its queries(add, get) in amortized O(1) time. The key word is amortized, which means that there are cases when it's not that fast. The worst case is O(n) in time for these operations, which you've probably dealt with in tc 28.
speedforces
Video: A very interesting Multiset Hashing Solution to Problem E
Posting this because the Editorial to problem E is not posted yet (and I talked to the problemsetters and their solution is different from mine)
This was the soln I implemented during testing. 158094993
For A, don't you mean $$$s_i = s_{n - i + 1}$$$ instead of $$$s_i = {n - i + 1}$$$?
In second problem ,AND SORTING i am confuse in why it is taking 'and(&)' of all elements present at not correct position, please help
We don't need to move the elements at the right places. Hence only the elements at wrong places should be eligible for swapping. Thus X should be & of all those values.
In part C https://codeforces.net/contest/1682/submission/158147851 if I don't write i<n-1 in the while loop, one of the test case fails on CF but passes in my device. Could someone explain why?
I'm afraid that I didn't catch the point of problem C.
Why don't we just make the subsequence in a monotonically increasing order?
Like this:[2 2 4 4 5 5].
By this way,LIS(a')=0.
Oh, I made such a stupid mistake. :( Neglect it.
Good Problem E!
Maybe Better for having a sooner editorial XD
Why not use Hash to solve the 1682A?
And why do you Hack unordered_map in 1682C?
Because its an overkill. Simpler easy to implement solns exist.
Because it's hackable.
I have added editorial of problem E. It's really long and detailed. Hope you would like it.
For problem E, how to understand the phrase "because we have to sort the permutation in a minimum number of moves which isn't possible if two cycles are merged at any instant".
I have understood, thanks.
In problem 1682C — LIS or Reverse LIS?, how do we have the answer 3 for this input:
1 1 2 3 3?1 3 2 3 1
Am I blind or what the ans in 13231 is still 2.Where r u getting an increasing sequence of 3
Can anyone explan the problem F?
Really good problem D and E! But why the editorial of problem F is really late......
We both are extremely busy in office, sorry for the delay. Will post it very soon.
Thanks really much!
Understand! Thanks very much!
Hey guys I have a question regarding C :
Consider the test case :
Now, there are three arrangements of $$$[1, 2, 2]$$$ :
So beauty in every case is $$$1$$$. My code outputs $$$1$$$ as well. However, the output and the formula from the editorial suggest the answer should be $$$2$$$. Can anybody point out where the flaw in my reasoning is?
Nevermind I made a mistake in case $$$[2, 1, 2]$$$ got it
I'm getting TLE at testcase #28 in Problem C. Here's my submission. Can anyone help me why I'm getting TLE.
Unordered maps can be blown up easily. You can refer comments above for the same.
LOL what made hash function slow here ? I'm new to this could you plz explain it.
https://codeforces.net/blog/entry/62393
hey, I am trying to understand why in problem C the solution is said to be NlogN? We simply add to the map and then traverse it, isn't it just N? what am I missing here?
Adding elements to std:map in C++ is log(size).
Alternate solution for problem E:
Firstly, observe that the edges corresponding to swaps required to sort one cycle form a tree which has all the nodes in the cycle (this is also present in the editorial, so I won't elaborate on it).
Now, let's consider how we can solve this subproblem:
There exists a tree of $$$n$$$ vertices, and an additional value $$$p_u$$$ for each node $$$u$$$. Now, some hidden order of edges was chosen, and then for each edge $$$(u, v)$$$, we swap the values of $$$p_u$$$ and $$$p_v$$$. You are given the final value of $$$p_u \quad \forall u \in [1, n]$$$. You need to recover any order of edges which would lead to the same values of $$$p$$$.
Note: For an edge $$$(u, v)$$$ when we remove this edge, the tree is split into two components, I will be calling the one containing $$$u$$$ as the $$$u$$$-component and the one containing $$$v$$$ as the $$$v$$$-component.
Claim 1: After all swaps, for every edge $$$(u, v)$$$, there exists exactly one node $$$x$$$ in the $$$u$$$-component, such that $$$p_y = x$$$ for some node $$$y$$$ in the $$$v$$$-component after all swaps (note that $$$(u, v)$$$ is an unordered pair for no loss of generality).
The $$$(u, v)$$$ swap occurs exactly once.
For all the swaps before the $$$(u, v)$$$ swap, no value can cross over from the $$$u$$$-component into the $$$v$$$-component, as that would imply one of the following:
During the $$$(u, v)$$$ swap, exactly one value goes to the $$$v$$$-component.
For all swaps after the $$$(u, v)$$$ swap, there is no interaction between the two components for the same reason as the case before the $$$(u, v)$$$ swap.
Now, to restore a valid order of swaps if any exists, we can simply do the following:
For any edge $$$(u, v)$$$ such that in the current state, the node $$$p_u$$$ is in the $$$v$$$-component and the node $$$p_v$$$ is in the $$$u$$$-component, add the swap of $$$(u, v)$$$ to the beginning of currently found order, and then swap $$$p_u$$$ and $$$p_v$$$.
I will just write an informal sketch of the proof for now and maybe update it later, I am too tired right now.
Consider the initial process in which the $$$p$$$ values on the edges were being swapped in the hidden order.
Let $$$f(u, v)$$$ be currently true if and only if in the current state, the node $$$p_u$$$ is in the $$$v$$$-component and the node $$$p_v$$$ is in the $$$u$$$-component.
We can observe that for any edge $$$(u, v)$$$, this property remains false until the edge gets swapped, when it becomes true.
It can be shown that it only becomes false afterwards if an edge of the form $$$(u, x)$$$ or $$$(v, y)$$$ is swapped.
Thus, our found order can be shown to always be valid (if a valid order exists) as for every node, we have the same relative order of swaps for every $$$(u, c)$$$ edge where $$$c$$$ is a neighbour of u.
This can be implemented pretty simply, by using some euler tour stuff.
Implementation: link
In problem C what will be the answer or how to sort this array
10 10 15 15 1 2 3 4 5