LeetCode 76. Minimum Window Substring

class Solution {
public:
    string minWindow(string s, string t) {
        unordered_map<char,int> hash; // char and the num it needs (it can be minus)
        int start=0;
        string res; int min_len=INT_MAX;
        for (char ch:t) hash[ch]++;    
        
        int count=0;
        for (int end=0;end<s.size();++end){
            if (hash.count(s[end])){
                --hash[s[end]];
                if (hash[s[end]]>=0) ++count;
                while (count==t.size()){
                    if (end-start+1<min_len){
                        min_len = end-start+1;
                        res = s.substr(start,end-start+1);
                    }
                    if (hash.count(s[start])){
                        ++hash[s[start]];
                        if (hash[s[start]]>0) --count;
                    }
                    ++start;
                }
            }
        }
        return res;
    }
};