Hello everyone, this is my editorial for Round #310. I decided to write one since the author of this round didn't have time to do it. Currently, there's only Div2A and Div2B available, and I'll try to write editorial for Div1A and Div1B soon (about Div1C and Div1D, [user:microtony,2015-06-28] has already written a nice tutorial for them (you can check out [here](http://mirror.codeforces.com/blog/entry/18923)), and Div1E is too difficult for me).↵
↵
UPD: Div1A is available now!↵
↵
[Div2A — Case of the Zeros and Ones](http://mirror.codeforces.com/problemset/problem/556/A)↵
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First, we can come up with a naive solution. While there's still some position i that 'remove-able' (`(s[i] = '0' and s[i+1] = '1')` or `(s[i] = '1' and s[i+1] = '0')`), we'll remove `s[i]` and `s[i+1]` from the string. However, in the worst case (contain all character `'0'` first, then character `'1'` or vise-versa), the complexity is **O(n^2)** and will surely got TLE.↵
↵
We should optimize this solution by using stack. Push each character in string s to the stack, and while pushing, check if the top two element of the stack is `('0' and '1')` or `('1' and '0')` (or we can simply write `abs(stack[top]-stack[top-1) = 1`). If this happen, remove them from stack (`top:=top-2`). The answer is the number of character left in the stack.↵
↵
_Solution using Stack_: [submission:11816506]↵
↵
Sound a bit too complicated for Div2A, right? There must be a more simple solution. We should notice that if there's still some zero and one in the string, there must be at least one position that 'remove-able'. So, we can remove one character `'0'` and one character `'1'` from the string until one type of character run out. We can come up with more simple solution with these analysis: just count the number of character `'0'` and `'1'` in the string, and the answer is `abs(cnt0-cnt1)`.↵
↵
_Solution with some analysis_: [submission:11816073]↵
↵
Both of the solution has complexity **O(n)**.↵
↵
[Div2B — Case of Fake Numbers](http://mirror.codeforces.com/contest/556/problem/B)↵
==================↵
↵
To solve this problem, first, we should notice that after `n` action, the gear will turn back to the initial state. So, if solution exist, the number of action to get it must be less than `n`. We can get an solution: just keep performing the action until we get the require structure (answer is `'Yes'` or the number of action goes equal `n` (answer is `'No'`). Complexity is **O(n^2)**.↵
↵
_My solution_: [submission:11816203]↵
↵
This solution is already fast enough to get AC. But we can easily optimize it further. We can notice that we will need `k = (n-a[1]) mod n` move to get a[1] equal to 0, so we can just calculate a[i] after `k` move (whick is `(a[i]-k+n) mod n, with i mod 2 = 0` and `(a[i]-k+n) mod n, with i mod 2 = 1`) and check if this is the required structure. Complexity is now **O(n)**.↵
↵
_The better solution_: [submission:11816999]↵
↵
[Div1A — Case of Matryoshkas](http://mirror.codeforces.com/contest/555/problem/A)↵
==================↵
↵
To make thing easier to explain, we will use 'Box' instead of 'Matryoshkas' (Now this problem's name should be 'Case of Box' :D)↵
↵
Consider the following test:↵
↵
~~~~~↵
7 3↵
2 1 2 4↵
3 3 5 6↵
1 7↵
~~~~~↵
↵
The following image demonstrate the test:↵
↵
↵
We know that we can put box `a` in box `b` only if box b doesn't contain any other boxes or contained in any other box. So, that mean box `b` must be alone. So, it would be make sense if we put all the box, which still inside some other box, out (in an appropriate order). However, we don't have to do this with the sequence `1, 2, ..., x` since nothing can be put in box `1`, and we can just put that sequence of box in box `x+1`. For example, with the above test, the process is described in below picture.↵
↵
↵
↵
↵
↵
We know that we can put box `a` in box `b` only if box b doesn't contain any other boxes or contained in any other box. So, that mean box `b` must be alone. So, it would be make sense if we put all the box, which still inside some other box, out (in an appropriate order). However, we don't have to do this with the sequence `1, 2, ..., x` since nothing can be put in box `1`, and we can just put that sequence of box in box `x+1` later. For example, with the above test, the process is described in below picture.↵
↵
↵
↵
This process take `n-(k-1)-x` moves.↵
↵
Then, we can put sequence of box `1, 2, ..., x` in box `x+1`, box `x+1` in box `x+2`, ... box `n-1` in box `n`. For example, with the above test, the process is described in below picture (again).↵
↵
↵
↵
↵
This process take `n-x` moves. ↵
↵
The final answer is `(n-(k-1)-x)+(n-x)` = `2*(n-k)-k+1`.↵
↵
_My solution:_ [submission:11821602]↵
↵
Complexity: **O(n)**
↵
UPD: Div1A is available now!↵
↵
[Div2A — Case of the Zeros and Ones](http://mirror.codeforces.com/problemset/problem/556/A)↵
==================↵
↵
First, we can come up with a naive solution. While there's still some position i that 'remove-able' (`(s[i] = '0' and s[i+1] = '1')` or `(s[i] = '1' and s[i+1] = '0')`), we'll remove `s[i]` and `s[i+1]` from the string. However, in the worst case (contain all character `'0'` first, then character `'1'` or vise-versa), the complexity is **O(n^2)** and will surely got TLE.↵
↵
We should optimize this solution by using stack. Push each character in string s to the stack, and while pushing, check if the top two element of the stack is `('0' and '1')` or `('1' and '0')` (or we can simply write `abs(stack[top]-stack[top-1) = 1`). If this happen, remove them from stack (`top:=top-2`). The answer is the number of character left in the stack.↵
↵
_Solution using Stack_: [submission:11816506]↵
↵
Sound a bit too complicated for Div2A, right? There must be a more simple solution. We should notice that if there's still some zero and one in the string, there must be at least one position that 'remove-able'. So, we can remove one character `'0'` and one character `'1'` from the string until one type of character run out. We can come up with more simple solution with these analysis: just count the number of character `'0'` and `'1'` in the string, and the answer is `abs(cnt0-cnt1)`.↵
↵
_Solution with some analysis_: [submission:11816073]↵
↵
Both of the solution has complexity **O(n)**.↵
↵
[Div2B — Case of Fake Numbers](http://mirror.codeforces.com/contest/556/problem/B)↵
==================↵
↵
To solve this problem, first, we should notice that after `n` action, the gear will turn back to the initial state. So, if solution exist, the number of action to get it must be less than `n`. We can get an solution: just keep performing the action until we get the require structure (answer is `'Yes'` or the number of action goes equal `n` (answer is `'No'`). Complexity is **O(n^2)**.↵
↵
_My solution_: [submission:11816203]↵
↵
This solution is already fast enough to get AC. But we can easily optimize it further. We can notice that we will need `k = (n-a[1]) mod n` move to get a[1] equal to 0, so we can just calculate a[i] after `k` move (whick is `(a[i]-k+n) mod n, with i mod 2 = 0` and `(a[i]-k+n) mod n, with i mod 2 = 1`) and check if this is the required structure. Complexity is now **O(n)**.↵
↵
_The better solution_: [submission:11816999]↵
↵
[Div1A — Case of Matryoshkas](http://mirror.codeforces.com/contest/555/problem/A)↵
==================↵
↵
To make thing easier to explain, we will use 'Box' instead of 'Matryoshkas' (Now this problem's name should be 'Case of Box' :D)↵
↵
Consider the following test:↵
↵
~~~~~↵
7 3↵
2 1 2 4↵
3 3 5 6↵
1 7↵
~~~~~↵
↵
The following image demonstrate the test:↵
↵
We know that we can put box `a` in box `b` only if box b doesn't contain any other boxes or contained in any other box. So, that mean box `b` must be alone. So, it would be make sense if we put all the box, which still inside some other box, out (in an appropriate order). However, we don't have to do this with the sequence `1, 2, ..., x` since nothing can be put in box `1`, and we can just put that sequence of box in box `x+1`. For example, with the above test, the process is described in below picture.↵
↵
↵
↵
↵
We know that we can put box `a` in box `b` only if box b doesn't contain any other boxes or contained in any other box. So, that mean box `b` must be alone. So, it would be make sense if we put all the box, which still inside some other box, out (in an appropriate order). However, we don't have to do this with the sequence `1, 2, ..., x` since nothing can be put in box `1`, and we can just put that sequence of box in box `x+1` later. For example, with the above test, the process is described in below picture.↵
↵
↵
↵
This process take `n-(k-1)-x` moves.↵
↵
Then, we can put sequence of box `1, 2, ..., x` in box `x+1`, box `x+1` in box `x+2`, ... box `n-1` in box `n`. For example, with the above test, the process is described in below picture (again).↵
↵
↵
↵
↵
This process take `n-x` moves. ↵
↵
The final answer is `(n-(k-1)-x)+(n-x)` = `2*(n-k)-k+1`.↵
↵
_My solution:_ [submission:11821602]↵
↵
Complexity: **O(n)**
