Find the longest string

class Solution
{
public:
    static bool cmp(string a, string b){
        if(a.length() != b.length()){return a.length() < b.length();}
        return a > b;
    }
    string longestString(vector<string> &words)
    {
        unordered_map<string, int> mp;
        int n = words.size();
        
        for(int i=0;i<n;i++){
            mp[words[i]]++;
        }
        
  // sort in this manner: 
  // i) first sort them on the basis of their length
  // ii) If their lengths are equal, sort them reverse lexicographically. 
        
        sort(words.begin(), words.end(), cmp);
        
        
        
        for(int i=n-1;i>=0;i--){
            int l = words[i].length();
           // ye is the variable used to check if all the 
           //prefixes of the word exists in the array
            int ye = true;
            string t = "";
            for(int k=0;k<l;k++){
                t += words[i][k];
                if(mp[t] == 0){ye = false; break;}
            }
            if(ye){return words[i];}
        }
        return "";
    }
};