Minimum Window Substring

//https://leetcode.com/problems/minimum-window-substring/

class Solution {
public:
    string minWindow(string s, string t) {
        
        
        
        map < char , int > mp1 , mp2 ; 
        int n = s.length() ; 
        int m = t.length() ; 
        if ( n < m )
                return "" ;
        
        for  ( int i = 0 ; i < m  ; i ++ )
            mp1[ t [ i ] ] ++ ; 
        
        
        int start = 0 , end = -1 ; 
        
        for ( int i = 0 ; i < n  ; i ++ )
        {
            mp2 [ s [ i ] ] ++ ; 
            int flag = 1 ;
            for ( auto j : mp1 )
            {   
                    if ( mp2 [ j.first ]  < mp1 [ j.first ] )
                    {
                        flag = 0 ; 
                        break ; 
                    }
            }
            if ( flag )
            {
                end = i ;
                break ; 
            }
        }
        if ( end == - 1)
                return "" ; 
        
        int len = end - start + 1 ; 
        string ans = s.substr ( start  , len ) ; 
        while ( start < n and end - start + 1 >= m  )
        {
            //cout << start << " " << end<< endl ; 
            if ( end - start + 1 < len )
            {
                 len =  end - start + 1 ; 
                 ans = s.substr ( start  , len ) ;  
                 //cout << start << " " << end << " " << ans << endl ; 

            }
            char c = s [ start ] ; 
            mp2 [ s [ start ] ] -- ; 
            start ++ ; 
            while ( mp2 [  c ]  < mp1 [ c ] and end < n )
            {
                
                end ++ ; 
                mp2 [ s [ end ] ] ++ ; 
            }
            
            if ( mp2 [ c ]  < mp1 [ c ] )
                    break ; 
        }
        
        
        return ans ; 
        
    }
};