HackerEarth Alex and his String problem solution

In this HackerEarth Alex and his String problem solution Alex has a string S of length N consisting of lowercase alphabets. He wants to find lexicographically smallest string X of length N that can be formed using the following operation.

In one operation, he can select any one character among the at most first K characters of string S, remove it from string S and append it to string X. He can apply this operation as many times as he wants.

Help Alex find the string X.

#include <bits/stdc++.h>
using namespace std;

int main() {
    priority_queue<char,vector<char>,greater<char>> pq;
    string s;
    cin>>s;
    int k; cin>>k;
    for(int i=0;i<k;i++) pq.push(s[i]);
    string ans;
    for(int i=k;i<s.length();i++){
        ans+=pq.top();
        pq.pop();
        pq.push(s[i]);
    }
    while(!pq.empty()) {
        ans+=pq.top();
        pq.pop();
    }
    cout<<ans<<"n";
    return 0;
}