HIIT Generation

input/code.cpp: In function 'int main()':
input/code.cpp:17:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < f.size(); i++) {
                         ~~^~~~~~~~~~

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

using namespace std;

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

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

Test 1

Verdict: ACCEPTED

Test 2

Verdict: TIME LIMIT EXCEEDED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED