Hello, Codeforces!
Hope you enjoyed the round! Editorials for all problems are out now. I hope this editorial be informative and helpful.
1557A - Ezzat and Two Subsequences
The average of a group of numbers always has a value between the minimum and maximum numbers in that group.
Since the average of a group of numbers always has a value between the minimum and maximum numbers in that group, it can be proved that the best approach to obtain the maximum sum of averages of two subsequences is to put the maximum number alone into one subsequence, and the rest of the numbers into the other subsequence. A proof by contradiction follows.
Assume a sorted array, so $$$a_1 \le a_2 \le \ldots \le a_n$$$. Assume that there exists a bigger answer if you take the two greatest numbers (instead of only one) in one subsequence. Therefore, we need to prove that:
$$$\sum_{i=1}^{n-1} a_i / (n - 1) + a_n < \sum_{i=1}^{n-2} a_i / (n - 2) + (a_{n-1} + a_n) / 2$$$
By simplifying the inequality:
$$$a_n < 2 / (n - 1) \cdot \sum_{i=1}^{n-2} a_i / (n - 2) + (n - 3) / (n - 1) \cdot a_{n-1}$$$
Assume $$$avg_1 = \sum_{i=1}^{n-2} a_i / (n - 2)$$$, so $$$a_1 \le avg_1 \le a_{n-2}$$$ as stated at the beginning of the tutorial. The inequality becomes:
$$$a_n < (2 \cdot avg_1 + (n - 3) \cdot a_{n-1}) / (n - 1)$$$
The right-hand side of the inequality is also an average $$$avg_2$$$, such that $$$avg_1 \le avg_2 \le a_{n-1}$$$ (which can be further simplified to $$$a_1 \le avg_2 \le a_{n-1}$$$).
This means that $$$a_n$$$ is strictly less than $$$avg_2$$$. In other words, it states that $$$a_n$$$ is strictly less than a certain number between $$$a_1$$$ and $$$a_{n-1}$$$. This is a contradiction as we stated at the beginning of the proof that the array is sorted ($$$a_n \ge a_{n-1} \ge \ldots \ge a_1$$$). Summing things up, taking at least one number along with the maximum number will never yield a greater answer.
1557B - Moamen and k-subarrays
You can ignore the $$$k$$$ given in the input, try to find the minimum $$$k$$$ you need to sort the array (let call it $$$mnK$$$).
If $$$mnK$$$ $$$\le$$$ $$$k$$$ then you can split some subarrays to make it equal $$$k$$$, so the answer is will be "YES". otherwise, the answer will be "NO".
You need to split the array into the minimum number of subarrays such that each subarray is sorted.
This problem can be solved for \textbf{at least} $$$k$$$ subarrays as it is easy to just add extra subarrays (if needed) to achieve \textbf{exactly} $$$k$$$ subarrays. To solve this problem, you need to know what are the numbers that can be grouped into the same subarray. This can be done by maintaining the sorted array along with the non-sorted array.
As the numbers are distinct, we can iterate over the non-sorted array, and just add each element $$$a_i$$$ to the subarray ending in $$$a_{i - 1}$$$ IFF they follow each other in the sorted array, or start a new subarray if they do not follow each other.
For example, if the (non-sorted) array is $$$[$$$ $$$2$$$, $$$3$$$, $$$ -1$$$, $$$1$$$], the sorted array will be $$$[$$$ $$$-1$$$, $$$1$$$, $$$2$$$, $$$3$$$ $$$]$$$. If we iterate over the non-sorted array, we will add $$$2$$$ to a new subarray, then we will add $$$3$$$ to the same subarray as they follow each other in the sorted array. After that, we will start a new subarray at $$$-1$$$ as $$$-1$$$ and $$$3$$$ do not follow each other in the sorted array. Finally, we will add $$$1$$$ to the subarray containing $$$-1$$$. It should end up like this: { $$$[$$$ $$$2$$$, $$$3$$$ $$$]$$$, $$$[$$$ $$$-1$$$, $$$1$$$ $$$]$$$ }.
Using this approach, you can get the smallest number of subarrays needed. If it is strictly greater than the given $$$k$$$, the answer is ''NO''. Otherwise, it is ''YES''.
1557C - Moamen and XOR
We don't care about the values in the array to know it's valid or not, we just need to know for every bit how many numbers have this bit on or off.
Try to build the array from the most significant bit ($$$k-1$$$) to the least significant bit $$$0$$$ using dynamic programming.
Can you optimize dynamic programming code with some combinatorics?
From now on, I will use $$$And$$$ to describe the result of the bitwise AND operation over all the elements in the array, and $$$Xor$$$ to describe the result of the bitwise XOR operation over all the elements in the array.
Let's call the array is valid if the value of $$$And$$$ $$$\ge$$$ $$$Xor$$$.
We don't care about the values in the array. We just need to know, for every bit, the number of indices at which this bit is on or off.
So let's build the array from the most significant bit ($$$k-1$$$) to the least significant bit ($$$0$$$) using dynamic programming and combinatorics optimization.
Define an array $$$dp$$$ where $$$dp_{i, equal}$$$ $$$=$$$ the number of ways to build the array from the $$$i$$$-th bit to $$$0$$$-th bit. $$$equal$$$ is $$$true$$$ if $$$And$$$ $$$=$$$ $$$Xor$$$ in the previous bits, and $$$false$$$ if $$$And$$$ $$$>$$$ $$$Xor$$$ ($$$And$$$ $$$<$$$ $$$Xor$$$ is not a valid state).
Our base case is $$$dp_{-1,0}$$$ $$$=$$$ $$$1$$$, and $$$dp_{-1,1}$$$ $$$=$$$ $$$1$$$.
If $$$equal$$$ is $$$false$$$ at any moment, then you can choose any subset of indices to contain $$$1$$$ in the $$$i$$$-th bit.
Therefore, $$$dp_{i,0}$$$ $$$=$$$ $$$2^n$$$ $$$\cdot$$$ $$$dp_{i-1,0}$$$.
Now if $$$n$$$ is odd there are $$$2$$$ possible choices:
- You can make $$$i$$$-th bit $$$=$$$ $$$1$$$ in an even number of indices, then $$$And_i$$$ will be $$$0$$$ and $$$Xor_i$$$ will be $$$0$$$ too.
- You can make $$$i$$$-th bit $$$=$$$ $$$1$$$ in all indices, then $$$And_i$$$ will be $$$1$$$ and $$$Xor_i$$$ will be $$$1$$$ too.
You Don't have any other valid choices. So $$$dp_{i,1}$$$ $$$=$$$ $$$dp_{i-1,1}$$$ $$$\cdot$$$ ({number of ways to choose number of indices from $$$n$$$} $$$+$$$ $$$1$$$). ($$$+1$$$ for the second choice).
If $$$n$$$ is even there are $$$2$$$ possible choices:
- You can make $$$i$$$-th bit $$$=$$$ $$$1$$$ in an even number (less than $$$n$$$) of indices, then $$$And_i$$$ will be $$$0$$$ and $$$Xor_i$$$ will be $$$0$$$ also.
- You can make $$$i$$$-th bit $$$=$$$ $$$1$$$ in all indices, then $$$And_i$$$ will be $$$1$$$ and $$$Xor_i$$$ will be $$$0$$$. and now $$$equal$$$ will be $$$false$$$.
You Don't have any other valid choices. So $$$dp_{i,1}$$$ $$$=$$$ $$$dp_{i-1,1}$$$ $$$\cdot$$$ (number of ways to choose number of indices from $$$n$$$ ) + $$$dp_{i-1,0}$$$.
We can precalculate the factorial, to get $$$nCr$$$ in $$$\mathcal{O}(1)$$$, and then calculate the number of ways to choose an even number from $$$n$$$ before starting the $$$dp$$$. The total complexity will be $$$\mathcal{O}(k + n)$$$.
1557D - Ezzat and Grid
Try to count the maximum number of rows that makes a beautiful grid, and remove the others.
Can you get some dynamic programming formula, and then optimize it with some ranges data structures?
We can use dynamic programming to get the maximum number of rows that make a beautiful grid.
Define the 2d array, $$$dp$$$, where $$$dp_{i,j}$$$ $$$=$$$ maximum number of rows (from row $$$1$$$ to row $$$i$$$) that make a beautiful grid, and has $$$1$$$ in column $$$j$$$ at the last row I have in the biggest beautiful grid. the last row in the biggest beautiful grid is the not necessary to be $$$i$$$
Form the definition:
$$$dp_{0,j}$$$ $$$=$$$ $$$0$$$.
$$$dp_{i,j}$$$ $$$=$$$ $$$1$$$ $$$+$$$ $$$\max_{k \in C_i}$$$ {$$$dp_{i-1,k}$$$} if $$$grid_{i,j}$$$ $$$=$$$ $$$1$$$.
Otherwise, if $$$grid_{i,j}$$$ $$$\neq$$$ $$$1$$$, then $$$dp_{i,j}$$$ $$$=$$$ $$$dp_{i-1,j}$$$ .
where $$$C_i$$$ is that set of columns that contain $$$1$$$ in row $$$i$$$.
As you know, the set $$$C_i$$$ contains the intervals, so we just search in some intervals for the maximum, or update some intervals in the previous layer in $$$dp$$$. We can do it faster using Segment tree.
So the algorithm will be as follows:
Define an array $$$prev$$$, where $$$prev_i$$$ $$$=$$$ the previous row of $$$i$$$ in which maximum beautiful grid end with $$$i$$$-th row. We will use it to get the rows that will not be removed.
Build a segment tree of pairs ($$$value$$$, $$$index$$$) initially with { $$$0$$$ , $$$-1$$$ }.
Then for each $$$i$$$ from $$$1$$$ to $$$n$$$:
- Get the maximum value in all the ranges $$$[l_j,r_j]$$$ that contains $$$1$$$ at the $$$i$$$-th row. Let's call it $$$mx$$$.
- Store $$$prev_{i}$$$ $$$=$$$ $$$mx.index$$$.
- Update all the ranges $$$[l_j,r_j]$$$ of this row like this: $$$seg_j$$$ $$$=$$$ $$$max($$$ $$$seg_j$$$ $$$,$$$ { $$$mx.value$$$ $$$+$$$ $$$1$$$ $$$,$$$ $$$i$$$ }).
- Finally, get the rows that have the maximum value using the $$$prev$$$ array, and remove the others.
The total complexity will be $$$\mathcal{O}(n + m\log{}10^9)$$$ or $$$\mathcal{O}(n + m\log{}m)$$$ if you make a coordinate compression to the values.
1557E - Assiut Chess
Try to force the king to move into one of the corners down.
If you put the queen in a row $$$x$$$, move the queen to the left of the row, then start swiping the row right, one square at a time. If you've visited all $$$8$$$ squares on the row and the king never made a vertical move, it means $$$|$$$ current king row $$$-$$$ current queen row $$$|$$$ $$$\ge 2$$$ (if the king were on row ($$$x + 1$$$\$$$x-1$$$), he would've been forced to move $$$1$$$ square (down\up) at some point).
This is one of many possible solutions. We need to force the king to move into one of the four corners (bottom right or bottom left corner in this solution) to ensure that the king will be trapped (cannot move anymore).
Place the queen on the top row. After the king makes a $$$1$$$ move, he should be below the queen's row.
Suppose the queen is on the row $$$x$$$ with the king below it (row $$$x$$$ $$$+$$$ $$$i$$$ where $$$i$$$ > $$$0$$$).
If $$$i$$$ $$$=$$$ $$$1$$$, we cannot move down to the next row as the king may move up and we will not be able to trap it. Otherwise, we can move down by one unit.
To ensure that the king is not on the next row, scan the current row, $$$x$$$, by moving the queen from the leftmost column to the rightmost column one square at a time. Therefore, you can move the queen as follows:
During the scan, if you have visited all $$$8$$$ squares of the current row and the king never made a vertical or diagonal move, it means that $$$i \ge 2$$$ and you can go down by one row. It is now guaranteed that the king is still below the queen.
If the king were on row $$$x$$$ $$$+$$$ $$$1$$$, he would have been forced to move $$$1$$$ square down at some point. If he ever goes down, move the queen down by one row.
If the king moves up, start scanning the row again. This can only happen a limited number of times without the king moving into a check.
In total, the queen needs to apply step $$$2$$$ up to $$$8$$$ times. At each row, step $$$1$$$ needs to be applied, so it takes $$$8$$$ moves. Step $$$1$$$ also needs to be applied every time step $$$3$$$ is applied, which can happen at most $$$8$$$ times. In total, that is $$$8$$$ $$$\cdot$$$ $$$(8$$$ $$$+$$$ $$$8)$$$ $$$=$$$ $$$128$$$ moves.
Auto comment: topic has been updated by AhmedEzzatG (previous revision, new revision, compare).
even though the round has many problems, but no one can disagree that the problems were great and beautiful.
So there is no explanation on how this wrong solution passed the system tests?
ok. It's maybe wrong. but it's just hard to make an Interactor counter all such solutions I guess. We tried to kill all the wrong solutions of the testers. actually, there were only 2 testers who could solve the problem.
Good idea,but weak interactor :(
I think using math formula in C more beautiful and easier than editorial
My solution using some binomial theorem and flt 125416610
It faster than editorial solution
can you explain what did you do?
$$$\binom{n}{0}+\binom{n}{2}+ \cdots = 2^{n-1}$$$
You can calculate this by using Binomial Theorem, which you subtract $$$(1+1)^n$$$ with $$$(1-1)^n$$$
However, there's an exception. When $$$n=0$$$ , the left part is 1. That's because $$$(1-1)^n$$$ will become undefined.
Don't ignore that during exam :)
explain please, I solved it pure math too, you can look at my 125416186 to see what things I already understood so that you could skip them if you want to.
Dude, your logic for when n is even was really funny. Good one!
can somebody tell me abt my WA https://codeforces.net/contest/1557/submission/125416469
m[max_ele] = 0
and 0 can be a valid element in your input array.
so thats reducing your cnt1 by 1 in test 3. and hence a WA.
If input array have duplicates your m[a[i]] will be over written and hence some data is lost this will produce an error suppose that array is 2 2 2 2 then always a[i]=2 and m[2] will be over written 3 times and eventually will store the last value and previous will be lost
Back to back segtree problems for the past 3 rounds.
Can you share the interactor as well? Would be interesting to know how does it decides where to move the king.
We try a lot of strategies for each cell, some greedy (for example, we try to make the king stay in the mid cells, Or try to go Up\downs if possible), and others random. maybe I will share the code later.
Randomized approach for E:
It seems that this solution uses far less than $$$130$$$ queries, or can you kill my solution?
my code
We can actually checkmate the king in 5 (4 if the queen stays out of the edges) moves by checkmating on the edge!
https://imgur.com/a/mwBV1Zk
I think that can get rid of the randomization part as if we place the queen at (4, 4), there are at most 31 squares the king can initially occupy. Thus, we have an upper bound of 4 * 31 = 124 queries.
However, in practice, the solution generally uses <10 queries.
Code: 125423506
Whoa this is genius O_o
Just in case this helps anyone, I tried to implement a cleaner and commented version of the code above. The zig-zag idea is really nice!
127869222
Can someone elaborate what are the leaves of seg tree used in D? If rows would have up to 10^5 columns the seg tree would be trivial, but with 10^9 I wasn't able to come up with seg tree structure and editorial didn't help neither.
Even the "dpi,j = maximum number of rows (from row 1 to row i) that make a beautiful grid, and has 1 in column j at the last row." doesn't make much sense to me, as there can be up to 10^9 j indexes.
yes they didn't wrote it explicit, but in the end they wrote you can use coordinate compression.
that would reduce columns from $$$10^9$$$ to $$$2m$$$.
you have segments: $$$[1,10],[2,20],[3,30]$$$
you can process them as $$$[1,4],[2,5],[3,6]$$$
i will still yeild the same answer for the problem.
and you only $$$\text{dp}[i-1]$$$ to compute $$$\text{dp}[i]$$$ . so you can use one single row of $$$2m$$$ size and use segment tree to maintain it. all m updates in $$$\log{n}$$$ time.
i did got the idea in the end and also implemented but i am getting WA at test 4.
if anyone can help be debug.
I used the same approach and got AC. I think you made a mistake in the update function. While updating the segment tree we need to check whether a higher value is updated to lower value. 125463800
thanks, yes i did that check with
ppgt()function which will propagate any changes that were made along the path we need for query/update, i missed a part inppgt()to reset current root's lazy value to null after upddting its children. silly mistake, took me hours to find out.Solution for first three questions. https://www.youtube.com/watch?v=vBqSLVoXYsI
Love this round. Hoping to have more lovely rounds like this in the future. (◍•ᴗ•◍)❤
My heart starts bitting faster every time I see 5 problems in a contest. I was at the top of my abilities to solve C today
SegmentTree for D is such an overkill.
Can you please brief your approach without segment tree for D.
Do you mean you can do it without segment tree?
We can get every set of rows $$$i$$$ that for each column $$$j$$$, $$$grid_{i,j}=1$$$. Then construct a DAG by connecting the edges of each row in the sets from the first to the last, and find the longest path on the graph. The comlexity is $$$O(n+m)$$$.
How did you do it without a log factor?
Yes you are right. It's difficult to get rid of a log factor on the work before DP. I mean you can find the longest path on the DAG in $$$O(n+m)$$$.
how does this work tho? doesnt longest path in dag have complexity O(V + E), how do you make sure there aren't too many edges cause then i would assume it becomes O(N^2). I looked at your code and it looks like there are only N edges but I don't get how something like that would work unless you are compressing the dag?
Can you explain how to generate the graph in O(n+m)?
Hello , I didn't get the logic behind 2nd problem .
https://ide.geeksforgeeks.org/8684Y9kbwQ
I'd suggest you clean out your code a little before asking for help!
Sorry .
You can check this one .
https://ide.geeksforgeeks.org/8684Y9kbwQ
I have cleaned unnecessary things.
What is your thought process exactly for this question?
Why do I get WA on pretest 2? https://codeforces.net/contest/1557/submission/125391145 Brief explanation: If $$$n$$$ is odd, the answer is just $$$(2^{n-1}+1)^k$$$. Otherwise, if $$$A_n(j)$$$ is the number of ways for the bits from 0 to $$$j$$$ such that $$$And\geq Xor$$$, I get the recursion $$$A_n(j+1)=2^{nj}+2^{n-1}A_n(j)$$$, which solves to $$$A_n(k)=2^{nk-k}+2^{kn-n+1}-2^{nk-n-k+2}$$$. Where is the flaw?
EDIT: found it, wrong recursion, it should be
AhmedEzzatG Do you mind looking into why this solution fails on D? I used the approach described in the editorial during the contest, but really have no clue why the 4th pretest is failing. It's giving me headache. Thanks.
125401562
Hi, sorry for delay. you have a small bug in segment tree code in line 68.
return tree[v] = max(query(2 * v, l, m, ql, qr), query(2 * v + 1, m + 1, r, ql, qr));it must be
return max(query(2 * v, l, m, ql, qr), query(2 * v + 1, m + 1, r, ql, qr));Thank you so much, I appreciate it. A dumb mistake that yet again cost me -200 delta.
Regardless, I liked D very much (the only problem I attempted, so I can't say for others) and I had a great time solving it, although it gave me some trouble. Thanks for the round!
really quick pure combinatorics sol for C:
We'll look at each bit independently. The bit for the LHS will be 1 iff all the bits are 1 and 0 otherwise. For the RHS, the bit will be 1 iff the number of bits that are ones is odd, and 0 otherwise. In order to handle both at the same time, you need to split into cases based on the parity of $$$n$$$.
Suppose $$$n$$$ is odd. Then, you can never have the case that the LHS is strictly greater than the RHS, because if the AND is equal to 1, then the number of bits that are 1 is odd, meaning the XOR is 1. Thus, you must have equality.
If both the LHS and RHS is 1, then it must be all ones so that is 1 way. Then if both sides are equal to 0, we must have exactly an even number of ones, or
which is equal to $$$2^{n-1}$$$. Thus, in the odd case, the answer is $$$\left(2^{n-1}+1\right)^k$$$.
Now suppose $$$n$$$ is even. This time, it is possible to have the strict inequality by taking all ones. You then must consider two cases: when we have equality for both sides and when we have the strict inequality.
We tackle the first case. Both bits can not both be equal to one, as that would require all the bits to be one, for an even total, and an odd number of ones, contradiction. Thus we need both to be 0, meaning not all bits are equal to 1, and we must have exactly an even number of ones. This corresponds to
Again, this can be seen to be equal to $$$2^{n-1}-1$$$ through whatever method you want. Thus, there's a total of $$$\left(2^{n-1}-1\right)^k$$$ arrays in this case.
Now suppose we have a strict inequaltiy. What we'll do is fix the bit where we have a 1 on the LHS and 0 on the RHS, and iterate over the positions for that bit: suppose the $$$i$$$'th bit will satisfy this. For the $$$i-1$$$ bits before it, we must have equality, which gives $$$\left(2^{n-1}-1\right)^{i-1}$$$ from the analysis of the previous case. Then, we have exactly 1 way to get the $$$i$$$'th bit, which is to take all ones. Then, the remaining $$$k-i$$$ bits don't matter, so there are $$$\left(2^n\right)^{k-i}$$$ choices here.
Putting both cases together and summing over all $$$i$$$, we have
different arrays in the case that $$$n$$$ is even. Compute this now however you want.
If you wanted a closed form for this, you can see that the summation is actually equal to
where you use the factorization
EDIT: sorry for the typoes oops as you can tell i am terrible at proofreading
hey, I implied your method: can you tell me, why I'm getting the wrong answer my submission: 125428860
could you please amend it as well?
if n is even,it should be
Yes, of course... thanx This dickhead has wasted my 3 hours posted the wrong solution. And I was thinking something is wrong with my modular template.
something is wrong with your mindset.
In the 4th paragraph, "we must have exactly an even number of zeroes" => "we must have exactly an even number of ones"
Nice comment, however be careful with the case: $$$2^{n - 1} + 1 \equiv 0 \mod M$$$.
In this problem it won't occur, but it could happen if $$$M = 998244353$$$, for example:
and the answer is $$$2^{n k} + k \cdot 2^{n(k - 1)}$$$ for this special case.
How did you conclude that for M = 1e9 + 7, ((2^(n-1) + 1) mod M) won't ever be 0. And why is the answer 2^{n k} + k * 2^{n(k — 1)} for this case?
Also, I'd be glad if anyone could recommend learning resources (preferably text) for the same.
compute all $$$2^n, n = 0, \cdots M - 1$$$ or some primary number theory.
You may need to learn
markdown,mathjaxortex.Use contradiction.Assume that there exists an $$$n$$$ such that
Assume that $$$n$$$ is the smallest root of that equation.
Assume that $$$m$$$ is the smallest root of the equation
It's easy to see that $$$m|10^9+6$$$ and $$$m\mid 2n$$$ but $$$m\nmid n$$$.
So we can know that $$$2|m$$$ and $$$\frac{m}{2}\mid n$$$ and $$$\frac{2n}{m}$$$ is an odd number.
It's easy to see that
However
So we can know that $$$2^\frac{m}{2}\equiv -1\bmod 10^9+7$$$.
According to $$$m|10^9+6$$$ we can know that $$$\frac{10^9+6}{m}$$$ is an odd number.
So we can know that
In other words
However
Contradiction.
Sorry for my bad English :(
hey, can u tell me that in your even n case, in sub-case of inequality, while setting i bit as most significant, for k-i bits answer should be (2^n)^(k-i) because we do not have to set any specific bit in all n numbers as same (in those k-i bits)..
and in your case of 2^(k-i), we imply that ith bit is same in all n numbers(in those k-i bits), which is not necessary..
Could you explain how exactly did you arrive at the closed form equation?
It's is a geometric series so one can get it by applying the formula for summing a geometric series and then simplifying; however I actually got it by working through all the steps:
Divide by the ratio of consecutive terms (one would normally multiply, but dividing seems simpler here):
Take the difference between the sum from 1 to k and the sum from 0 to k-1:
Simplify:
could some one tell me why this is wrong https://codeforces.net/contest/1557/submission/125421372
You could have a contiguous subarray but you might have a value that splits that contiguous array into parts E.g- 1 2 3 7 4 and k=2 your solution would give yes but the answer is no because 1.2.3.7 is contiguous but 4 needs to be put in between 3,7 so it needs 3 subarrays
I think it's meaningless to set the space of problem D so tight…… It's hard for $$$O(m\log10^9)$$$ to get pass, since it's easily getting MLE.
But also great problems, thanks for the contest!
Discretization please! Codeforces Round NEVER set space limits tightly!
But a time complicity of $$$\mathcal{O}(n + m\log{}10^9)$$$ was given by the problem writer as a accepted one.
I think
#define int long longis the culprit here. Maybe try removing that line and see it fits inside the memory limit or not.I have a different solution for E.
In the beginning the possible positions for KING is the whole chessboard. And if the king do a move , some positions will become impossible. So you can do $$$30$$$ random walk , to make the possible positions become very few(may be greater than one).
And for each possible position , you try to checkmate it .
You can do it for a known position in at most 8 steps :
And remember to maintain the possible positions after every move .
It seems that it use only about total $$$40$$$ moves for a test case.
code
we cannot rely on random walks what if the king is oscillating back and fro in two squares and doing random walks yr queen doesn't give check I know that probability of this is less but is not 0 so this approach can fail once in a million
In fact if you try to checkmate a position , you can also find some positions impossible .
So I think it is impossible to be hacked .
if the king only oscillates in two squares then the possible positions will be 64-16=48 and if u are unlucky enough u can exhaust 130 moves
I mean if you try to checkmate the positions one by one , you will continue finding some impossible positions .
yaa makes sense
I find the interactor for E a little "dumb": in all testcases, I made no more than 20 moves to kill the king lol (if the output of the testcase is the number of moves)
Thanks a lot for a detailed editorial.
problem E was super cute and I recommend everyone to just take a look at it :)
Well, D is a good problem. I solved many problems that can directly construct schemes through dp arrays before. But problem D ISN'T such problem, so I got Wrong answer on pretest 4. This is a good reminder to me that the record transfer is used to construct the scheme, rather than backward pushing according to the dp array.
But I'm still disappointed that I didn't passed D in the round. I am so close to the correct solution!
thank you for the nice editorial
What would be the rating of Div2C question?
1700 maybe
AhmedEzzatG Thank you, Sir, The problems were really Good, We can learn a lot from them.
i did this in D problem: if the first and second row have something in common(which I am precomputing) then we can just move on to next row and set previous column as current index and if they don't have anything common, then we have to find the minimum of the two cases: removing the first row and removing the second row. So I call for two functions, with previous values as current index for one and the previous index for the other.
Recursive Code:
But I am doing isMerging step in O(n), can we do it in less than that? Also, I was not able to print the rows deleted.
on what concept problem c is based upon.?
Dynamic Programming and Combinatorics
hello everyone In Problem B is showing runtime error in testcase 2 but the code seems to be fine have a look guys. https://codeforces.net/contest/1557/submission/125441413 kindly help if you can. Thanks
even if n==k u have to take whole input :-)
Problem C Video Editorial
I solved it in contest and I show my thought process so it might help some of you
Can someone explain my WA? All my tests is AC, but here WA on pretest 2. https://codeforces.net/contest/1557/submission/125395144
try for
1
4 3
2 4 3 5
it should print "NO"
I understand, thank you.
How my approach is wrong for the submission of 1557B — Moamen and k-subarrays problem ? Please help 125447732
try for
1
4 3
2 4 3 5
it should print 'NO'
Hey, In problem A we proved that taking the maximum number in one subsequence is better than taking the two greatest numbers (instead of only one) in one subsequence.My question is how we can generalize this argument for the whole problem as there might be the other cases except for taking the two greatest number.AhmedEzzatG
I proved it a bit differently:
Suppose the array is split into 2 sub-sequences: $$$A$$$ with $$$k$$$ elements and sum $$$S_A$$$, and $$$B$$$ with $$$n-k$$$ elements and sum $$$S_B$$$, and further assume that $$$k \leq n-k$$$.
Step 1. We can show that if $$$x \in A$$$ and $$$y \in B$$$, then $$$x \geq y$$$.
Proof by contradiction. Assume that $$$y > x$$$, then we can swap $$$x$$$ and $$$y$$$ to get a larger result:
Step 2. If $$$A$$$ has more than 1 element, we can move the smallest element of $$$A$$$ to $$$B$$$ and increase the result. Let $$$x$$$ be the smallest element of $$$A$$$. By the previous step, $$$x$$$ is larger than any element of $$$B$$$, so after moving $$$x$$$ from $$$A$$$ to $$$B$$$ both the average of $$$A$$$ and average of $$$B$$$ will increase.
I had the same problem with the editorial proof, but I easily understood yours. Truly elegant. Thanks
One interesting observation — if both subsequences have the same length ($$$k = n - k$$$) then the result does not depend on how we do the split and is equal to twice the total average.
Thanks for the help.You are amazing!!
The proof is not so trivial to be arrived at during the contest for div2 problem A. So, I think, most of the participants just made a guess and hoped for the best.
On the other hand, it's still problem A of div2, so if you get a hunch and go over some examples to "convince" yourself that it's the right observation, it's probably OK to trust your intuition and submit fast, and only try to prove it formally later.
A correction — the statement in the first step is true only if $$$k$$$ is strictly less than $$$n - k$$$ (otherwise $$$\frac 1 k - \frac 1 {n-k} = 0$$$).
If $$$k = n - k$$$ the result is the same no matter how we split, and if $$$x \geq y$$$ for any $$$x \in A$$$, $$$y \in B$$$ it is as good as any other split.
Problem C，forgot to add enough mod,and failed twice.100 points T T
In problem D,why adding $$$ dp[i][j] = max_{k\in C_i} {dp[i - 1][k]} $$$ step as call
modify(1,1,N - 1,mx)is useless and wrong? In my opinion,tutorial's definition points out that $$$ grid[i][j] = 1 $$$ so the transfrom $$$ dp[i][j] = max_{k\in C_i} {dp[i - 1][k]} $$$ is not exist.And that's why addmodify(1,1,N - 1,mx)will casue wrong?:/In Problem E, this is a false promise:
after 30+ submissions of getting RTE and TLE I realized that my code making a move of the type : $$$(6, 1)$$$ -> $$$(6, 1)$$$. I should have got WA verdict but didn't.
But I got a "Wa" verdict. 125371395
It is a bit random. I got many TLEs. There is no reason I should get TLE.
I also wasted a lot of my time trying to debug why my code was getting TLE, when it should be getting WA. Honestly, this E should have been tested more, I don't know how it got published with such a weak and inconsistent interactor
I can't understand "You can make i-th bit = 1 in an even number of indices, then Andi will be 0 and Xori will be 0 too." only one position is 1, then the xor could 1 why is 0?
maybe it means you choose even number of 1
For Problem C, I think the editorial is wrong because $$$dp[i][0]$$$ is just a product of $$$2^n$$$ which corresponds to all possible cases.
Let's define two vectors of size $$$k$$$ :
Then for allcases:
And for dp:
I got WA on test 43 125484134 , test 43 seems random...., can somebody tell me why
I just change min() to max() and AC,why
I dont know what someone expected for when trying to copy code to on top of the leaderboard of virtual participation. Lmao
I hope that each example of the problems can be more valuable.
In the solution of D it said:
What does it mean here "last row"? Is it the row number $$$i$$$ or it is the last row of the biggest beautiful grid?
If it is the row number $$$i$$$ then the following doesn't make sense. It should be just 0.
Otherwise $$$dp_{i, j}$$$ should somehow depend on $$$j$$$, but the right hand side doesn't contain $$$j$$$ in any way
The tutorial of problem D truly has confused me a lot. Now I've figured it out:
The last row is not necessarily $$$i$$$ or $$$i-1$$$, it means the last row of the biggest beautiful and in that row at index $$$j$$$ it stores $$$1$$$.
The Otherwise part is also not correct. It should be $$$dp_{i,j}=dp_{i-1,j}$$$
Why is this blog getting so many negative votes, Codeforces community doesn't appreciate the authors' effort!
That's just because the terrible interactor of problem E.
I don't know any other reasonable causes.
I'm really sorry that this happened but we did everything we could to make all the wrong solutions fail. But it is difficult to find a general way for all solutions. But now I've worked on it and it's a bit better. We apologize once again for that.
Because the tutorial is badly written... and some of the problems' pretest is tremendously STRONG.
In the tutorial of problem D, the following statement is not correct:
Should be: If $$$grid_{i,j}\not=1$$$, $$$dp_{i,j}=dp_{i-1,j}$$$.
Yes, I am confused about this, thanks.
Sorry for this mistake, I have modified it now.
Is the solution for problem A wrong? I think we can not sum things up like that
What do you mean?
If we devide sequence to the biggest number and the others. He proved that moving second biggest num to the group of biggest number wont have better solution. But this does not mean its true for other cases, I know there is another solution, you can read comments above
You can generalize the proof to any size of the sequence, not just 2. However, I think the proof in this comment is better than the editorial proof.
The first edi that has -100 downvotes :/
This is not the worst, the worst is the announcement of a contest with several hundred downvotes :/
Can somebody please help me figure out, why my code, which quite literally implements the solution described in the editorial (except that I am scanning from left to right instead of top to bottom), not work? It gives runtime error because my queen somehow enters the 8th column, whereas it should be able to checkmate while it is in the 7th column. Any help in figuring out the error (either in the solution or the implementation) would be appreciated.
Why so much hate around this contest? It wasn't bad at all, C and D were interesting enough and the other tasks were good, maybe only E wasn't finally prepared but it's not the point to hate the whole thing :\ what a heck
AhmedEzzatG I don't understand the following part:
If equal is false at any moment, then you can choose any subset of indices to contain 1 in the i-th bit.You could select the odd number of indices to contain the i-th bit and then And would become less than Xor.
If $$$equal$$$ $$$=$$$ false that mean $$$And$$$ > $$$Xor$$$ in the previous bits. Therefore, whatever the value of $$$And$$$ or $$$ Xor $$$ in i-bit, $$$And$$$ will remain greater than $$$Xor$$$.
I have a solution to problem E that only needs 16 moves in worst case, verified by DFS.
https://codeforces.net/contest/1557/submission/125862124
I believe it can be made better, but that means slower runtime and will be hard to verify.
A simple random solution for E: 126166286. Just do random moves and try to keep a list of valid starting positions for the king each time. When just one starting cell remained, simply trap the king.
Why this entry has so many downvotes?
About B,I use map<int,int> nxt,directly link the next value,but I failed Test3 in 58th case,I don't know why,anybody can find the mistakes in may code?It will help me a lot,thanks so much!
I used a different approach in C, then given in editorial. If n is odd, then for each i from 1 to k, either the ith bit should be on for all the n numbers or the number of on bits should be even so that the XOR does not become one. If n is even, we will follow similar approach but there are some extra cases. For each i from 1 to k, we will count the number of cases where number of on bits for each i should be even(but less than n). Then for each i from 1 to k, consider that the ith bit in all n numbers is on and there is no j from i+1 to k such that the jth bit is on in all n numbers. All the bits less than i can be on or off in all n numbers. All this counting can be done using simple combinatorics. You can look at my solution here https://codeforces.net/contest/1557/submission/144063292 Sorry for the text wall.
Hi guys, i can't understand what wrong with my solution for C. Please help me. My solution:here.I found my mistake.
what's your mistake?
mathematically incorrect solution
Is there a way to solve D without coordinate compression ?