Idea: Vladosiya Prepared by: Vladosiya

**Tutorial**

Tutorial is loading...

**Solution**

```
def solve():
n, m = map(int, input().split())
a = input()
ans = 0
for ch in range(ord('A'), ord('H')):
ans += max(0, m - a.count(chr(ch)))
print(ans)
for _ in range(int(input())):
solve()
```

Idea: senjougaharin Prepared by: senjougaharin

**Tutorial**

Tutorial is loading...

**Solution**

```
def solve():
n, f, k = map(int, input().split())
f -= 1
k -= 1
a = list(map(int, input().split()))
x = a[f]
a.sort(reverse=True)
if a[k] > x:
print("NO")
elif a[k] < x:
print("YES")
else:
print("YES" if k == n - 1 or a[k + 1] < x else "MAYBE")
t = int(input())
for _ in range(t):
solve()
```

1980C - Sofia and the Lost Operations

Idea: senjougaharin Prepared by: Gornak40

**Tutorial**

Tutorial is loading...

**Solution**

```
#include <stdio.h>
#include <stdbool.h>
#define MAXN 200200
#define MAXM 200200
int n, m, k;
int arr[MAXN], brr[MAXN], drr[MAXM], buf[MAXN];
int cmp_i32(const void* pa, const void* pb) {
return *(const int*)pa - *(const int*)pb;
}
void build() {
k = 0;
for (int i = 0; i < n; ++i) {
if (arr[i] != brr[i])
buf[k++] = brr[i];
}
qsort(buf, k, sizeof(*buf), cmp_i32);
}
bool check() {
for (int i = 0; i < n; ++i)
if (brr[i] == drr[m - 1])
return true;
return false;
}
bool solve() {
if (!check()) return false;
qsort(drr, m, sizeof(*drr), cmp_i32);
int ib = 0, id = 0;
while (ib < k && id < m) {
if (buf[ib] == drr[id])
++ib, ++id;
else if (buf[ib] < drr[id])
return false;
else ++id;
}
return ib == k;
}
int main() {
int t; scanf("%d", &t);
while (t--) {
scanf("%d", &n);
for (int i = 0; i < n; ++i)
scanf("%d", arr + i);
for (int i = 0; i < n; ++i)
scanf("%d", brr + i);
scanf("%d", &m);
for (int j = 0; j < m; ++j)
scanf("%d", drr + j);
build();
if (solve()) printf("YES\n");
else printf("NO\n");
}
}
```

**Tutorial**

Tutorial is loading...

**Solution**

```
#include <bits/stdc++.h>
using namespace std;
bool good(vector<int>&b){
int g = __gcd(b[0], b[1]);
for(int i = 1; i < int(b.size()) - 1; i++){
int cur_gcd = __gcd(b[i], b[i + 1]);
if(g > cur_gcd) return false;
g = cur_gcd;
}
return true;
}
bool solve(){
int n;
cin >> n;
vector<int>a(n);
for(int i = 0; i < n; i++){
cin >> a[i];
}
int g = -1;
int to_del = -1;
for(int i = 0; i < n - 1; i++){
int cur_gcd = __gcd(a[i], a[i + 1]);
if(cur_gcd < g){
to_del = i;
break;
}
g = cur_gcd;
}
if(to_del == -1) return true;
vector<int>b0 = a, b1 = a, b2 = a;
if(to_del > 0) b0.erase(b0.begin() + to_del - 1);
b1.erase(b1.begin() + to_del);
if(to_del < n - 1) b2.erase(b2.begin() + to_del + 1);
return good(b0) || good(b1) || good(b2);
}
int main(){
int t;
cin >> t;
while(t--){
cout << (solve() ? "YES" : "NO") << "\n";
}
}
```

1980E - Permutation of Rows and Columns

Idea: senjougaharin Prepared by: senjougaharin

**Tutorial**

Tutorial is loading...

**Solution**

```
#include <iostream>
#include <vector>
#include <algorithm>
#include <set>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
vi read_ints(int n) {
vi res(n);
for (int i = 0; i < n; ++i) {
cin >> res[i];
}
return res;
}
vvi read_matrix(int n, int m) {
vvi res(n);
for (int i = 0; i < n; ++i) {
res[i] = read_ints(m);
}
return res;
}
void solve() {
int n, m;
cin >> n >> m;
vvi a = read_matrix(n, m), b = read_matrix(n, m);
int nm = n * m;
vi pos1i(nm), pos2i(nm), pos1j(nm), pos2j(nm);
for (int i = 0; i < n; ++i) {
for (int j = 0; j < m; ++j) {
int x = a[i][j] - 1;
int y = b[i][j] - 1;
pos1i[x] = pos2i[y] = i;
pos1j[x] = pos2j[y] = j;
}
}
vector<set<int>> pi(nm), pj(nm);
for (int x = 0; x < nm; ++x) {
int i1 = pos1i[x], i2 = pos2i[x];
int j1 = pos1j[x], j2 = pos2j[x];
pi[i1].insert(i2);
pj[j1].insert(j2);
}
for (int x = 0; x < nm; ++x) {
if (pi[x].size() > 1 || pj[x].size() > 1) {
cout << "NO\n";
return;
}
}
cout << "YES\n";
}
int main() {
int t;
cin >> t;
for (int _ = 0; _ < t; ++_) {
solve();
}
return 0;
}
```

1980F1 - Field Division (easy version)

Idea: MikeMirzayanov Prepared by: Vladosiya

**Tutorial**

Tutorial is loading...

**Solution**

```
#include <bits/stdc++.h>
#define int long long
#define pb emplace_back
#define mp make_pair
#define x first
#define y second
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
typedef long double ld;
typedef long long ll;
using namespace std;
mt19937 rnd(time(nullptr));
const ll inf = 1e9 + 1;
const ll M = 998244353;
const ld pi = atan2(0, -1);
const ld eps = 1e-6;
bool cmp(pair<int, int> &a, pair<int, int> &b){
if(a.x != b.x) return a.x > b.x;
return a.y < b.y;
}
void solve(int tc){
int n, m, k;
cin >> n >> m >> k;
vector<pair<int, int>> a(k);
map<pair<int, int>, int> idx;
for(int i = 0; i < k; ++i){
cin >> a[i].x >> a[i].y;
idx[a[i]] = i;
}
sort(all(a), cmp);
vector<int> ans(k);
int total = 0;
int cur = m + 1;
int last = n;
for(auto e: a){
if(cur > e.y) {
ans[idx[e]] = 1;
total += (cur - 1) * (last - e.x);
cur = e.y;
last = e.x;
}
}
total += (cur - 1) * last;
cout << total << "\n";
for(int e: ans) cout << e << " ";
}
bool multi = true;
signed main() {
int t = 1;
if (multi)cin >> t;
for (int i = 1; i <= t; ++i) {
solve(i);
cout << "\n";
}
return 0;
}
```

1980F2 - Field Division (hard version)

Idea: MikeMirzayanov Prepared by: Vladosiya

**Tutorial**

Tutorial is loading...

**Solution**

```
#include <bits/stdc++.h>
#define int long long
#define pb emplace_back
#define mp make_pair
#define x first
#define y second
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
typedef long double ld;
typedef long long ll;
using namespace std;
mt19937 rnd(time(nullptr));
const ll inf = 1e9 + 1;
const ll M = 998244353;
const ld pi = atan2(0, -1);
const ld eps = 1e-6;
bool cmp(pair<int, int> &a, pair<int, int> &b){
if(a.x != b.x) return a.x > b.x;
return a.y < b.y;
}
void solve(int tc){
int n, m, k;
cin >> n >> m >> k;
vector<pair<int, int>> a(k);
map<pair<int, int>, int> idx;
for(int i = 0; i < k; ++i){
cin >> a[i].x >> a[i].y;
idx[a[i]] = i;
}
idx[{0, 0}] = k++;
a.emplace_back(0, 0);
sort(all(a), cmp);
vector<int> ans(k);
vector<int> total(k + 1), cur(k + 1, m + 1), last(k + 1, n);
for(int i = 1; i <= k; ++i){
auto e = a[i - 1];
total[i] = total[i - 1];
cur[i] = cur[i - 1];
last[i] = last[i - 1];
if(cur[i] > e.y) {
ans[idx[e]] = 1;
total[i] += (cur[i] - 1) * (last[i] - e.x);
cur[i] = e.y;
last[i] = e.x;
}
}
cout << total[k] << "\n";
for(int i = 1; i <= k; ++i){
auto e = a[i - 1];
if(ans[idx[e]] == 0)continue;
int tot = total[i - 1];
int cr = cur[i - 1];
int lst = last[i - 1];
for(int j = i + 1; j <= k; ++j){
auto ee = a[j - 1];
if(cr > ee.y){
tot += (cr - 1) * (lst - ee.x);
cr = ee.y;
lst = ee.x;
}
if(ans[idx[ee]] == 1){
ans[idx[e]] = tot - total[j];
break;
}
}
}
ans.pop_back();
for(int e: ans) cout << e << " ";
}
bool multi = true;
signed main() {
int t = 1;
if (multi)cin >> t;
for (int i = 1; i <= t; ++i) {
solve(i);
cout << "\n";
}
return 0;
}
```

1980G - Yasya and the Mysterious Tree

Idea: Gornak40 Prepared by: Gornak40

**Tutorial**

Tutorial is loading...

**Solution**

```
#include <bits/stdc++.h>
using namespace std;
struct trie {
int l, c;
vector<array<int, 2>> node;
vector<int> cnt;
trie(int l, int max_members) : l(l), c(0), node((l + 2) * max_members + 3), cnt((l + 2) * max_members + 3) {}
void add(int x) {
int cur = 0;
for (int i = l; i >= 0; --i) {
bool has_bit = (1 << i) & x;
if (!node[cur][has_bit]) {
node[cur][has_bit] = ++c;
}
cur = node[cur][has_bit];
++cnt[cur];
}
}
void remove(int x) {
int cur = 0;
for (int i = l; i >= 0; --i) {
bool has_bit = (1 << i) & x;
if (!node[cur][has_bit]) {
node[cur][has_bit] = ++c;
}
cur = node[cur][has_bit];
--cnt[cur];
}
}
int find_max(int x) {
int cur = 0, ans = 0;
for (int i = l; i >= 0; --i) {
bool has_bit = (1 << i) & x;
if (node[cur][!has_bit] && cnt[node[cur][!has_bit]]) {
ans += 1 << i;
cur = node[cur][!has_bit];
}
else {
cur = node[cur][has_bit];
}
}
return ans;
}
};
const int N = 2e5 + 2;
int x[N];
bitset<N> dp;
vector<array<int, 2>> e[N];
void dfs(int c, int p) {
for (auto [i, w] : e[c]) {
if (i == p) {
continue;
}
dp[i] = !dp[c];
x[i] = x[c] ^ w;
dfs(i, c);
}
}
void solve() {
int n, m, gx = 0;
cin >> n >> m;
for (int i = 1; i <= n; ++i) {
e[i].clear();
}
for (int i = 1, u, v, w; i < n; ++i) {
cin >> u >> v >> w;
e[u].push_back({v, w});
e[v].push_back({u, w});
}
dfs(1, 0);
vector<trie> t(2, trie(30, n));
for (int i = 1; i <= n; ++i) {
t[dp[i]].add(x[i]);
}
while (m--) {
char c;
cin >> c;
if (c == '^') {
int y;
cin >> y;
gx ^= y;
}
else {
int a, b;
cin >> a >> b;
t[dp[a]].remove(x[a]);
int same_group = t[dp[a]].find_max(x[a] ^ b);
int diff_group = t[1 - dp[a]].find_max(x[a] ^ b ^ gx);
t[dp[a]].add(x[a]);
cout << max(same_group, diff_group) << "\n";
}
}
}
int main() {
ios_base::sync_with_stdio(0); cin.tie(0);
int t;
cin >> t;
while (t--) {
solve();
}
return 0;
}
```

Why is system testing so slow these days?

Because there are so many people participating in contests

I guess it's due to the horrible 150+ tests of problem C... Most solutions took ~500ms on each test, which means that everyone who passed problem C(a large amount!) will take ~75 seconds to be judged

Sometimes I wonder if it would be better to include anti-unordered data in the pretests. This might help avoid the pointless but time-consuming hack judging?

I guess they shouldn't add all the the hacks to the main test cases. The C problem has 150 test cases but most of them are for Unordered Map. They can try ignoring similar purposed hacks by categorizing them or something.

Unordered map is constant time right, why is that giving TLE

Is it because the constant is higher for the test case. Clarify if you can why this code gave TLE 263947136

Yeah so the average case of an unordered map is O(1) for accessing data, but due to collisions the worst case for accessing data is O(N), so the solution becomes O(N^2) instead of O(N). You can read this article- https://codeforces.net/blog/entry/62393

Thanks

its because unordered_map uses hashing and in worst case hashing haa O(n) time complexity,thats why its giving tle. use map always in place of using unorderd_map

How does one get to know that a test is for unordered map?

Most of these hacks use a pretty standard generator, it is designed/works in a way that it hacks all the solutions using some form of a hash table, be it an unordered map ,unordered set or a python dictionary.

C took 100 lines of code for me :(

It could be done in 20 lines ig

Here is a much simpler solution for Problem C.

The last element of modification array has to be present in D. All the elements which are different in B from A have to be present in D as well to carry out the modification. Except for that any other element in D does not matter because you could be changing that element in A to an intermediate number but then you finally change it to what it is in B.

E could be easily solved using set of sets: Solution Link

Why does it work?

It is just different implementation of same logic as in editorial.

bro we wrote literally the same code. :)

wait for the next div3 4 to participate again

what you gonna do about it ? BOMB me ?? MOHAmmada LMAO

Be respectful bro...

.

testcasesforces

can someone explain why only checking the rows is not enough in E.

For example consider the test case of 3×3 Matrix as

A :

1 2 3

4 5 6

7 8 9

B :

2 3 1

4 5 6

7 8 9

We cannot transform A to B using any row or column swaps

F is Mike's idea :O

But I think you can't avoid

`sort`

so the time complexity can't be $$$\mathcal O(n)$$$.My sol for F:

Find all corners. Remove all corners and run the algorithm for a second time to get a second group of corners.

A fountain becomes a

newcorner (after one of the old corners is removed) if and only if:coveredby only one corner.A corner $$$(u,v)$$$ covers the range {$$$(x,y)|x\in[1,u],y\in[v,m]$$$}.

A fountain becomes a new corner after removing the corner covers it.

It's easy to calculate change of area now.

264008397

(Why

`$$$\{$$$`

-> $$${$$$)Do you know why my code for F1 doesn't work?

264033779

You can't find corners correctly in this way.

Can you please elaborate on what you mean? Thanks!

Radix_Sort by

`((int64_t)x << 32) | (INT32_MAX - y)`

The most important thing was to explain that the part after sorting is linear. (and my memory so short that I forgot about sorting by the end of tutorial)

Can any one help me to understand problem c.? And send solution easly

Our goal is to find whether, given a list of sequential operations, an array could be translated into other or not.

The only operations that matter here are when an element is changed between a[i] -> b[i]. We ensure all such operations (say,

`d_k`

) are actually present in d.Since d is also sequential, for any successful transformation, all

`d_k`

must be present at the end of d. As for a simple solution, kindly refer one from _Runtime__Terror_ a bit above in this discussion.Thanks.

why I have to remove a[i] in problem d? can someone pls explain with one example.

let b be an array and for each index i in b :b[i] = gcd(a[i],a[i+1]) .

when you remove a[i] the array a changes => array b changes

dang,how much longerr would the system testing go.

Can anyone tell me, in C, why dm must be present in b? I read the question 50 times, still didn't understood.

if not , the (index) you apply dm at in the last operation will differ from b[index] as dm != b[t] for any (t) ( if dm is not present in b )

Thank You!

since you have to apply all operations in the order they are given, then dm will always need to be applied last and there is no following operation that can replace it. Therefore it should appear in the final array

Thank You!!

YES NO forces

Can someone explain why my submission 263986368 on problem c got a tle but something like this 263962017 didn't?

same, now i just read that unordered_map with weak hash fxns can cause collisions in testcases and end up in O(n^2), it was safe to use normal map in this problem

Both solutions which I've mentioned use Hashmap.

used unordered map in Problem C and got TLE in system tests and delta changed to -ve.

Got hacked on C, i guess its newbie time

Need Help!! I am beginner, there are times when i feel my approach is correct based on my intuition but i fail to prove it why, in such cases what should i do, try implementing it in code or give some more time to prove my intuition?

problem d, solution 1 — 263972949 failed system test (TLE), solution 2 — 264171599 passed all the tests, code — same in both letter by letter, only difference — solution 1 submitted on Python3 while solution 2 submitted on PyPy3, result — got a "-1" in place of a "+"

Problem C solved in C, coincidence or intentional?

Can someone explain why my submission 263980440 on problem C got TLE but this submission 264168646 didn't?

count in multiset is briefly O(n)

The time complexity of the count() operation in a multiset in C++ is O(log(n) + k), where n is the size of the multiset and k is the number of occurrences of the element being counted. I think TLE occurs when there is a testcase where k is very large.

Yes, so effectively it is O(n). Got hacked for the same reason T_T

E can be done with Zobrist Hashing 264181127.

For problem E on the basis of the solution, at the end, after sorting both matrix A and B on the basis of rows as well as on the basis of columns, both should turn out to be equal then only our answer is YES. So to simplify my implementation i took the sum of each rows in A and in B in two vectors and finally after sorting checked if both come out to be same and i did same for the columns also, since the sum property should also satisfy since the numbers are unique but i am getting WA on test 2. Can anyone explain what's wrong with my given implementation. I am not able to think on which test case my implementation will fail. please help 264209981

unique numbers does not guarantee a unique sum. (1,4) snd (2,3) have the same sum value, it is bound to give you wrong answer.

came up with a wierd soln for E https://codeforces.net/blog/entry/130129

When I submitted problem 3 in the contest it showed accepted but right now it is showing TLE on test 10. Can anyone please explain why does this happen ? It has happened with me once before. (I am new to codeforces, so i don't know about all the rules)

You got hacked. (your code failed system testing) This is probably because you used unordered_map or something in your code. See the note at the end on editorial for C

thanks

Video Editorial for D

https://youtu.be/O2-wL5OXyYc

When is contest rating generally updated after the contest and in how many days editorial of the contest come out?

Resources to learn trie?

https://medium.com/basecs/trying-to-understand-tries-3ec6bede0014

This has a good explanation

Trie problems

thanks bro

Can someone please tell me the mistakes in my solution to problem E (https://codeforces.net/contest/1980/submission/264239803)? I have used a simple sorting-based method.

In problem F2 why the following code isn't giving TLE? What is time complexity of the loop for given problem ?

The complexity is k^2 which obviously TLEs for k <= 2*10^5

That was accepted. But now I got it why. Because we are only checking points between corners the time complexity will be O(n).

D was talking about removing i-1 i or i+1 then why is it removing the from 0 to i or 0 to i-1 or 0 to i+1. If it is works the same then can someone explain me how?

In Problem E: Suppose for all those test cases which have the answer "YES", we are also asked to output the possible actions (not necessarily the minimum) in the form: i) r a b -> swap row a with b. ii) c a b -> swap column a with b. Then, what would the approach be for this one?

Good idea! I still confused this way , which found a deatialed path to transform matrix B , from contest time to now.

Problem E is flexible and great mind!The Tuorial only introduce solved way.It's so upset>_<.Can Anyone explain that detailed the reason of way >_< ?

you can check mine it is relatively easy i first checked row wise. there i made a map for every row after sorting it and then checked for every sorted row in b if it existed in the original map. If it doesnt exist then no() else we check further .similarly for columns https://codeforces.net/contest/1980/submission/264333535 .

Thank you for your reply! I incline to figure out how transform A into B , such as proof. The tutorial is understood by me , but i still don't why , specifically.

I used multiset , which is slower than your way. I think your idea better than mine!

can someone explain the editorial code of C

In my perspective , it's maybe complex for you. https://codeforces.net/contest/1980/submission/264339347 It's my submission. I hope this hopeful for you!

I Was confused because of the compare function because it's not bool but turned out it's just normal sorting

Editorial code for C can also be hacked because

`qsort`

worst case is $$$O(n^2)$$$: https://codeforces.net/contest/1980/hacks/1033165Code`Unexpected verdict`

means that one of the solutions marked on Polygon as Correct can't pass this test. Vladosiya Gornak40I have no ideas why I got wrong answer on problem G: https://codeforces.net/contest/1980/submission/264341580. My solution is the same as editorial.

Yahia_Talaat88

Can somebody explain why it doesn't give TLE? I tried fixing each using rowswap and colswap and checked in the end if both matrices are equal or not. Submission

I am calculating that same sum of row and columns present in the array B or not

https://codeforces.net/contest/1980/submission/264546235

I am unable to find where it is failing. Can you tell some test case where it fail.

I don't think sum is an ideal hashing method.

Read YES, expected NO.

Hope it helps:)

264169959

why it is giving wrong answer for problem D.

Can anyone help why my code for 1980G - Yasya and the Mysterious Tree is not working?

264619004

264609607

I am happy if any of the above two works.

in problem E I have reached till a point that I want to check if one vector is a permutation of another and all of them must be shifted the same amount is this correct or just give up

A O(nm) solution for E that uses xor hashes:

Submissionhttps://codeforces.net/contest/1980/submission/265038082

In G why do you need to remove the x[a] first in order to get the result?

can anyone point out what went wrong in my code problem c ;

## include <bits/stdc++.h>

using namespace std; int original[210101] ; int found[210101] ; int diffs[210101] ; int modifier[210101] ; int main() { int t ; cin >> t ; while (t--) { int n ; cin >> n ; for (int i = 0 ; i < n ; i++) cin >> original[i] ; ///// int j = 0 ; for (int i = 0 ; i < n ; i++) { cin >> found[i] ; if (original[i] != found[i]) { diffs[j] = found[i] ; j++ ; } }

}

Did someone solve problem G using Centroid Decomposition?

Solved E via union find, was way more intuitive for me, but basically the same concept as AC.

Submision: 265923059 https://codeforces.net/contest/1980/submission/265923059

can anyone tell any testcase where my code fails? (https://codeforces.net/contest/1980/submission/266299334)