Minimum Window Substring

    Solution: Use two pointers: start and end. First, move 'end'. After finding all the needed characters, move 'start'.

 string minWindow(string S, string T) {
        int N=S.length();
        int M=T.length();
        if(N<M) return "";
        int need[256]={0};
        int find[256]={0};
        for(int i=0;i<M;i++)
            need[T[i]]++;
        int count=0,mStart=-1,mEnd=N;
        int start=0,end=0;
        for(;end<N;++end){
            if(find[S[end]]<need[S[end]]){
                count++;
            }
            find[S[end]]++;
            if(count==M){
                while(find[S[start]]>need[S[start]]||need[S[start]]==0){
                    find[S[start]]--;
                    start++;
                }
                if(end-start<mEnd-mStart){
                    mStart=start;
                    mEnd=end;
                }
            }
        }
        return mStart==-1?"":S.substr(mStart,mEnd-mStart+1);
    }