minWindow

    string minWindow(string s, string t) {
        vector<int> hash(128); // ASCII 0-127;
        int match = 0, start = 0, end = 0;
        int pos = start, len = INT_MAX; // return s.substr(pos, len);
        for (char c : t) hash[c]++;
        while (end < s.length()) {
            
            
            // Means it's one of the char in t
            if (hash[s[end]] > 0) { match++; }            
            // When end walks, minus every element by 1 
            hash[s[end]]--;
            end++;
            
            while (match == t.length()) {
                if (end - start < len) {
                    len = end - start;
                    pos = start;
                }
                
                // Means it's one of the char in t (it's reduced to 0 when end pointer walks through).
                if (hash[s[start]] == 0) { match--; }
                // When start walks, plus every element by 1
                hash[s[start]]++;
                start++;
            }
        }
        
        return len == INT_MAX ? "" : s.substr(pos, len);
    }