最小覆盖字串（滑动窗口）

题目：给两个字符串，请你在S中找到包含T中全部字母的最短字串。

#include <iostream>
#include <cstring>
#include <algorithm>
#include <unordered_map>

using namespace std;

string min_window(string s, string t)
{
    unordered_map<char, int> need, window;
    
    for(char c: t) need[c]++;
    
    int left = 0, right = 0;
    int valid = 0;
    
    int start = 0, len = 0x3f3f3f3f;
    
    while(right < s.size())
    {
        char c = s[right];
        right++;
        if(need.count(c))
        {
            window[c]++;
            if(window[c] == need[c]) valid++;
        }
        
        while(valid == need.size())
        {
            if(right - left < len)
            {
                start = left;
                len = right - left;
            }
            char d = s[left];
            left++;
            if(need.count(d))
            {
                if(window[d] == need[d]) valid--;
                window[d]--;
            }
        }
    }
    return len == 0x3f3f3f3f? "": s.substr(start, len);
    
}


int main()
{
    string s = "ADBECFEBBVANC";
    string t = "BANC";
    cout << min_window(s, t) << endl;
    return 0;
}