76. Minimum Window Substring

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".

S: 窗口型two poniter
用hashmap记录target中元素出现次数，用count记录总的target元素出现情况

    string minWindow(string &source, string &target) {
       unordered_map<char, int> map;
       int count = t.size();
       string result = "";
       if (count == 0) return result;
       for (char i : t) {
           map[i]++;
       }
       for (int i = 0, j = 0; j < s.size(); ++j) {
           if (map.find(s[j]) != map.end()) {
               if (map[s[j]] > 0) {
                   count--;
               }
               map[s[j]]--;
           }
           while (count == 0) {
               if (result.size() == 0 || result.size() > j - i + 1) {
                   result = s.substr(i, j - i + 1);
               }
               if (map.find(s[i]) != map.end()) {
                   if (map[s[i]] == 0) {
                       count++;
                   }
                   map[s[i]]++;
               }
               i++;
           }
       }
       return result;
    }