Minimum covering substring of two pointers

A daily topic ing, Today is a day hard topic 76. Minimum Window Substring

class Solution {
    
public:
    string minWindow(string s, string t) {
    
        vector <int> num(128,0);
        vector <bool> flag(128,false);

        for(int i=0;i<t.length();i++){
    
            flag[t[i]]=true;
            num[t[i]]++;
        } 

        bool fflag=true;
        int l=0,min_l=0,min_size=s.length()+1,cnt=0;
        for(int i=0;i<s.length();i++){
    
            if(flag[s[i]]){
    
                fflag=false;
                num[s[i]]--;
                if(num[s[i]]>=0){
    
                    cnt++;
                }
                while(cnt==t.length()){
    
                    if(i-l+1<min_size){
    
                        min_size=i-l+1;
                        min_l=l;
                    }
                    if(flag[s[l]]&&++num[s[l]]>0){
    
                        cnt--;
                    }
                    l++;
                }
            }
        }
        if(min_size<t.length()||min_size>s.length()||fflag){
    
            return "";
        }
        return s.substr(min_l,min_size);
    }
};