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".

class Solution {
public:
    string minWindow(string s, string t) {
        vector<int> map(128, 0);
        for(auto c:t) map[c]++;

        int b = 0, e = 0, dis = INT_MAX, head = 0;
        int counter = t.size();

        while(e < s.size()){
            if(map[ s[e++] ]-- > 0)  counter--;

            while(counter == 0){
                if(e - b < dis) {
                    dis = e-b;
                    head = b;
                }
                //剩下的不是t中的字符在map中的次数 <= 0
                if(map[ s[b++] ]++ == 0 )   counter++;
            }
        }
        return dis==INT_MAX? "":s.substr(head, dis);
    }
};