1925C - Did We Get Everything Covered - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
#define ll long long int
int main(){
ll t;
cin>>t;
while(t--){
    ll n,k,m;
    cin>>n >>k >>m;
    string s;
    cin>>s;
    set<char>s1;
    ll count=0;
    ll j=0;
    string w="";
    for(ll i=0;i<m;i++){
      s1.insert(s[i]);
      if(s1.size()==k){
        count++;
        s1.clear();
        w+=s[i];
      }
    }
    // cout<<s.size()<<endl;
    if(count>=n){
      cout<<"YES"<<endl;
    }
    else{
      cout<<"NO"<<endl;
    // we will constuct string of length n
    cout<<w;
    char c;
    for(ll i=97;i<(97+k);i++){
      bool ok=false;
      if(s1.find((char)i)==s1.end()){
          c=(char)i;
          ok=true;
          break;
      }
      
      if(ok){
        break;
      }
      
    }
    for(ll i=1;i<=(n-w.size());i++){
       cout<<c;
    }
    cout<<endl;
   }
}
}