[leetcode]Minimum Window Substring

题目：

Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).

For example,
S = "ADOBECODEBANC"
T = "ABC"

Minimum window is "BANC".

Note:
If there is no such window in S that covers all characters in T, return the emtpy string "".

If there are multiple such windows, you are guaranteed that there will always be only one unique minimum window in S.

解题：

　　 string minWindow(string S, string T) 
    {
		if(S.size() == 0)return "";
		if(S.size() < T.size())return "";
		int expect[128] = {0};
		int appear[128] = {0};
		int i;
		int cnt = 0;
		int start = 0;
		int min_start = 0;
		int min_length = INT_MAX;
		for(i = 0; i < T.size(); i++){
			expect[T[i]]++;
		}
		for(i = 0; i < S.size(); i++){
			if(expect[S[i]] > 0){
				appear[S[i]] ++;
				if(appear[S[i]] <= expect[S[i]])
					cnt++;
			}
			if(cnt == T.size()){
				while(appear[S[start]] > expect[S[start]]
				|| expect[S[start]] == 0){
					appear[S[start]]--;
					start++;
				}
				if(min_length > (i - start + 1)){
					min_length = i - start + 1;
					min_start = start;
				}
			}
		}
		if(min_length == INT_MAX)return "";
		return S.substr(min_start,min_length);
	}

　　30ms，还有优化的空间。