算法描述：

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).

Example:

Input: S = "ADOBECODEBANC", T = "ABC"Output: "BANC"

Note:

• If there is no such window in S that covers all characters in T, return the empty string "".
• If there is such window, you are guaranteed that there will always be only one unique minimum window in S.

解题思路：子字符串问题，可以采用强大的滑动窗口方法解决。首先用map对字符串进行映射，并用两个指针分别指向窗口左边和右边，通过不断的滑动两个指针实现对窗口内字符的操作。

string minWindow(string s, string t) {        string res = "";        unordered_map
     
       map;        for(char c:t) map[c]++;        int left=0;        int right = 0;        int distance = INT_MAX;        int count = t.size();        int head = 0;        while(right < s.size()){            if(map[s[right++]]-- >0) count--;            while(count==0){                if(right-left< distance){                    head = left;                    distance = right-head;                }                if(map[s[left++]]++ ==0) count++;            }        }        return distance==INT_MAX?"":s.substr(head,distance);    }