Substring
hdu-6948
Problem Description
You are given a string S[1…N] containing only lowercase letters. Now you need to find the longest substring S[l…r] such that every letter (a to z) appears no more than K times in the substring. You just need to output the length (r−l+1) of the longest substring.
Input
There are multiple test cases.
Each test case contains one integer K (1≤K≤N) and one string S in one line.
It’s guaranteed that the sum of lengths of the input strings is no more than 4×105.
Output
For each test case, print one integer in one line, denoting the length of the longest substring.
Sample Input
1 abcabcabc
2 abcabcabc
2 aaabbbccc
Sample Output
3
6
4

题解：尺取法遍历，长度为满足条件的最大长度，遍历下一个元素就将上个元素删掉。

#include<bits/stdc++.h>
using namespace std;

char c[9000000];
int book[30];

int n,m;

int main()
{
	while(cin>>n>>c)
	{
		int sum=0;
		int l=strlen(c);
		for(int a=0, b=0;a<l;a++)
		{
			while(a<=b&&b<l&&book[c[b]-'a']<n)
			{
				book[c[b]-'a']++;
				b++;
			}
			sum=max(sum,b-a);
			book[c[a]-'a']--;
		}
		cout<<sum<<endl;
	}
}