len

class Solution {
public:
    string minWindow(string s, string t) {
        int s_len=s.length();
        int t_len=t.length();
        
        vector<int> freq_t(256,0);
        vector<int> freq_s(256,0);
        for(int i=0;i<t_len;i++)
            freq_t[t[i]]++;
        
        int win_left=0; //Current window will always be [win_left,i]
        int ans=INT_MAX,ans_left; //Track smallest window
        int count=0; //For tracking number of characters encountered of string t in string s
  
  
        for(int i=0;i<s_len;i++){
            if(freq_t[s[i]]>0 && freq_t[s[i]]>freq_s[s[i]])
                count++;
            freq_s[s[i]]++;
            if(count==t_len){ 
                /*
    All characters required are in current window
                Now , try to minimize window by removing extra character at the start of window
                */
    
                while(win_left<i && //Extra frequency or zero frequency characters
                      (freq_t[s[win_left]]<freq_s[s[win_left]] || freq_t[s[win_left]]==0)){
                    
                    if(freq_t[s[win_left]]<freq_s[s[win_left]])
                        freq_s[s[win_left]]--;
                    win_left++;
                }
                if(ans>(i-win_left+1)){
                    ans=(i-win_left+1);
                    ans_left=win_left;    
                }
            }
        }
        if(ans==INT_MAX)
            return "";
        return s.substr(ans_left,ans);
    }
};