codeforces#P1609E. William The Oblivious
William The Oblivious
Description
Before becoming a successful trader William got a university degree. During his education an interesting situation happened, after which William started to listen to homework assignments much more attentively. What follows is a formal description of the homework assignment as William heard it:
You are given a string $s$ of length $n$ only consisting of characters "a", "b" and "c". There are $q$ queries of format ($pos, c$), meaning replacing the element of string $s$ at position $pos$ with character $c$. After each query you must output the minimal number of characters in the string, which have to be replaced, so that the string doesn't contain string "abc" as a subsequence. A valid replacement of a character is replacing it with "a", "b" or "c".
A string $x$ is said to be a subsequence of string $y$ if $x$ can be obtained from $y$ by deleting some characters without changing the ordering of the remaining characters.
The first line contains two integers $n$ and $q$ $(1 \le n, q \le 10^5)$, the length of the string and the number of queries, respectively.
The second line contains the string $s$, consisting of characters "a", "b" and "c".
Each of the next $q$ lines contains an integer $i$ and character $c$ $(1 \le i \le n)$, index and the value of the new item in the string, respectively. It is guaranteed that character's $c$ value is "a", "b" or "c".
For each query output the minimal number of characters that would have to be replaced so that the string doesn't contain "abc" as a subsequence.
Input
The first line contains two integers $n$ and $q$ $(1 \le n, q \le 10^5)$, the length of the string and the number of queries, respectively.
The second line contains the string $s$, consisting of characters "a", "b" and "c".
Each of the next $q$ lines contains an integer $i$ and character $c$ $(1 \le i \le n)$, index and the value of the new item in the string, respectively. It is guaranteed that character's $c$ value is "a", "b" or "c".
Output
For each query output the minimal number of characters that would have to be replaced so that the string doesn't contain "abc" as a subsequence.
Samples
9 12
aaabccccc
4 a
4 b
2 b
5 a
1 b
6 b
5 c
2 a
1 a
5 a
6 b
7 b
0
1
2
2
1
2
1
2
2
2
2
2
Note
Let's consider the state of the string after each query:
- $s = $"aaaaccccc". In this case the string does not contain "abc" as a subsequence and no replacements are needed.
- $s = $"aaabccccc". In this case 1 replacements can be performed to get, for instance, string $s = $"aaaaccccc". This string does not contain "abc" as a subsequence.
- $s = $"ababccccc". In this case 2 replacements can be performed to get, for instance, string $s = $aaaaccccc". This string does not contain "abc" as a subsequence.
- $s = $"ababacccc". In this case 2 replacements can be performed to get, for instance, string $s = $"aaaaacccc". This string does not contain "abc" as a subsequence.
- $s = $"bbabacccc". In this case 1 replacements can be performed to get, for instance, string $s = $"bbacacccc". This string does not contain "abc" as a subsequence.
- $s = $"bbababccc". In this case 2 replacements can be performed to get, for instance, string $s = $"bbacacccc". This string does not contain "abc" as a subsequence.
- $s = $"bbabcbccc". In this case 1 replacements can be performed to get, for instance, string $s = $"bbcbcbccc". This string does not contain "abc" as a subsequence.
- $s = $"baabcbccc". In this case 2 replacements can be performed to get, for instance, string $s = $"bbbbcbccc". This string does not contain "abc" as a subsequence.
- $s = $"aaabcbccc". In this case 2 replacements can be performed to get, for instance, string $s = $"aaacccccc". This string does not contain "abc" as a subsequence.
- $s = $"aaababccc". In this case 2 replacements can be performed to get, for instance, string $s = $"aaacacccc". This string does not contain "abc" as a subsequence.
- $s = $"aaababccc". In this case 2 replacements can be performed to get, for instance, string $s = $"aaacacccc". This string does not contain "abc" as a subsequence.
- $s = $"aaababbcc". In this case 2 replacements can be performed to get, for instance, string $s = $"aaababbbb". This string does not contain "abc" as a subsequence.