leetcode76

class Solution {
public:
    string minWindow(string s, string t) {
        string ans;
        int n = s.size();
        int minLen = INT_MAX;
        int index = -1;
        if (s.size() < t.size()) return ans;
        unordered_map<char, int> hashT;
        unordered_map<char, int> hashS;
        for (char c : t) ++hashT[c];
        for (int i = 0, j = 0; j < n; ++j) {
            ++hashS[s[j]];
            while (checkValid(hashS, hashT)) {
                int len = j - i + 1;
                if (len < minLen) {
                    minLen = len;
                    index = i;
                }
                if (--hashS[s[i]] == 0) hashS.erase(s[i]);
                ++i;
            }
        }
        return minLen == INT_MAX ? "" : s.substr(index, minLen);
    }
    bool checkValid(unordered_map<char, int>& hashS, unordered_map<char, int>& hashT) {
        for (auto item : hashT) {
            if (hashS.find(item.first) == hashS.end()) return false;
            if (hashS[item.first] < item.second) return false;
        }
        return true;
    }
};