76. Minimum Window Substring (Map)

class Solution {
public:
    bool ifContain(map<char,int>& source, map<char,int>& target) //check if S contains T
    {
        for(map<char,int>::iterator it = target.begin(); it != target.end(); it++) //map的遍历
        {
            if(source[it->first] < it->second) return false;
        }
        return true;
    }

    string minWindow(string S, string T) {
        int minLength = INT_MAX;
        int start = 0, end = 0, minStart = 0, minEnd = 0;
        map<char,int> source;
        map<char,int> target;
        source[S[start]]++; //map的插入[法I]source[key]=value; [法II]source.insert(make_pair(key,value));
        for(int i = 0; i< T.length(); i++)
        {
            target[T[i]]++;
        }
        while(1)
        {
            if(ifContain(source, target)){
                if(end-start+1 < minLength)
                {
                    minStart = start;
                    minEnd = end;
                    minLength = end-start+1;
                    if(minLength == T.size()) return S.substr(minStart,minLength);
                }
                source[S[start]]--; //寻找更小的窗口
                start++; 
            } 
            else //不包含，则扩大窗口
            {
                end++;
                if(end==S.size()) break;
                source[S[end]]++;
            }
        }
        if(minLength>S.size()) return "";
        else return S.substr(minStart,minLength);
    }
};