From index 0 to size-1, start = 0; end = 0; end move until 满足条件，然后记得把start在hash里清掉，继续移动start.

class Solution {
public:
    /*
     * @param source : A string
     * @param target: A string
     * @return: A string denote the minimum window, return "" if there is no such a string
     */
    string minWindow(string &source , string &target) {
        // write your code here
        vector<int> hashS (256, 0);
        vector<int> hashT (256, 0);
        int start = 0;
        int end = 0;
        int minLength = INT_MAX;
        string result = "";
        //if (source.size() < target.size()) {
        //    return "";
        //}
        if (source == "" && target == "") {
            return "";
        }
        for (int i = 0; i < target.length(); i++) {
            hashT[target[i]]++;
        }
        for (;start < source.length(); start++) {
            while (end < source.length() && !isValid(hashS, hashT)) {
                hashS[source[end]]++;
                end++;
            }
            if (isValid(hashS, hashT)) {
                if (end - start < minLength) {
                result = source.substr(start, end - start);
                minLength = end - start;
                }
            }
            if (hashS[source[start]] > 1) {
                hashS[source[start]]--;
            } else {
                hashS[source[start]] = 0;
            }
        }
        return result;
    }
    bool isValid(vector<int> &hashS, vector<int> &hashT) {
        for (int i = 0; i < 256; i++) {
            if (hashS[i] < hashT[i]) {
                return false;
            }
        }
        return true;
    }
};