双指针（Two Pointer）Template

public int findSubstring(string s){
        vector<int> map(128,0);
        int counter; // check whether the substring is valid
        int begin=0, end=0; //two pointers, one point to tail and one  head
        int d; //the length of substring

        for() { /* initialize the hash map here */ }

        while(end<s.size()){

            if(map[s[end++]]-- ?){  /* modify counter here */ }

            while(/* counter condition */){

                /* update d here if finding minimum*/

                //increase begin to make it invalid/valid again

                if(map[s[begin++]]++ ?){ /*modify counter here*/ }
            }

            /* update d here if finding maximum*/
        }
        return d;
    }