HIIT Generation

#include <iostream>
#include "stdio.h"

using namespace std;

int main() {
    char c;
    int n;
    cin >> n;
    string s = "HIIT";
    string f = "HIIT";
    f.reserve(1000000);
    for (int _ = 0; _ < n; _++) {
        cin >> c;
        for (int i = f.size() - 1; i > -1; i--) {
            if (f[i] == c) {
                f.replace(i, 1, s);
            }
        }

    }
    printf("%s\n", f.c_str());
    return 0;
}

Test 1

Verdict: TIME LIMIT EXCEEDED

Test 2

Verdict: TIME LIMIT EXCEEDED

Test 3

Verdict: TIME LIMIT EXCEEDED

Test 4

Verdict: ACCEPTED