Period

#include <vector>
#include <iostream>
#include <string>


using namespace std;


int main() {
    string s;
    cin >> s;

    int n = s.size();

    int *z = new int[n];
    for(int i = 0; i < n; i++) z[i] = 0;


    for(int i = 1; i < n; i++) {
        int j = 0;
        while (j + 1 < n && s[j + i] == s[j]) {
            j++;
        }
        z[i] = j;
    }

    /*
    for(int i = 0; i < n; i++) {
        cout << z[i] << " ";
    }
    cout << endl;
    */

    for(int i = 0; i < n; i++) {
        int l = n - i;
        if (z[i] >= l) {
            cout << s.substr(0, i) << endl;
            return 0;
        }
    }
    cout << s << endl;

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: TIME LIMIT EXCEEDED

Test 9

Verdict: TIME LIMIT EXCEEDED

Test 10

Verdict: TIME LIMIT EXCEEDED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED