Cod sursă (job #681099)

#include <bits/stdc++.h>
#define L 500005
using namespace std;
ifstream fin("bart.in");
ofstream fout("bart.out");

string s;
int pi[L];

void KMP(){
  pi[0] = 0;
  int k = 0;
  for (int i = 1; i < (int)s.size(); i++){
    while (k > 0 && s[k] != s[i])
      k = pi[k - 1];
    if (s[k] == s[i])
      k++;
    pi[i] = k;
  }
}

int main(){
  fin >> s;
  KMP();
  int ans = s.size();
  for (int i = 2; i <= (int)s.size(); i++)
    if (pi[i - 1] != 0 && i % (i - pi[i - 1]) == 0 && pi[s.size() - 1] > i - pi[i - 1])
      ans = i;
  for (int i = 0; i < ans - pi[ans - 1]; i++)
    fout << s[i];
  fout << "\n";
  return 0;
}