Period

#include<bits/stdc++.h> 
using namespace std; 
const int N = 1e5+1;
int n,z[N]; 
string s;


int main() 
{ 
    cin >> s;
    n = s.size();
    z[0]=n; 
    
    int j=1,k; 
    for (int i=1;i<n;) 
    { 
        if (j<i) j=i; 
        while (j<n && s[j]==s[j-i]) j++; 
        z[i]=j-i; 

        k=i+1;
        // i, .. k, ... j 
        // assign to following chars
        while (k+z[k-i]<j) {
            z[k]=z[k-i];
            k++;
        }
        i = k;
    } 

    for (int i=1;i<n;i++) {
        int j = i;
        while(n-z[j] == j) {
            j += z[j];
            if (j > n-1) {
                cout << s.substr(0, i);
                return 0;
            }
        } 
    }
    cout << s;
    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: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

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