Period

input/code.cpp: In function 'int main()':
input/code.cpp:14:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 1; i < str.size(); ++i) {
                   ~~^~~~~~~~~~~~

#include <vector>
#include <string>
#include <algorithm>
#include <unordered_set>
#include <iostream>

int main(void)
{
  std::string str;
  std::cin >> str;

  int shortest = 1;

  for (int i = 1; i < str.size(); ++i) {
    if (str[i - shortest] == str[i]) {
      continue;
    } else {
      shortest = i + 1;
    }
  }

  std::cout << str.substr(0, shortest) << std::endl;
  return 0;
}


#if 0
template <typename T>
void print_array(std::vector<T> const &v) {
  for (auto x: v) {
    std::cout << x << " ";
  }

  std::cout << std::endl;
}

// Small modification to return also the
// sorted array of valid lengths
void zarr(std::string const &s, std::vector<int> &z, std::vector<int> &lengths) {
  int n = s.size();
  int x = 0, y = 0;
  std::unordered_set<int> lengths_set;

  for (int i = 1; i < n; ++i) {
    z[i] = std::max(0, std::min(z[i - x], y - i + i));

    while (i + z[i] < n && s[z[i]] == s[i + z[i]]) {
      x = i; y = i + z[i]; z[i]++;
    }

    if (z[i] != 0) {
      lengths_set.insert(z[i]);
    }
  }

  for (int l : lengths_set) {
    lengths.push_back(l);
  }

  std::sort(lengths.begin(), lengths.end());
}

int main(void) {
  std::string str;
  std::cin >> str;
  std::vector<int> z(str.size());
  std::vector<int> lengths;

  zarr(str, z, lengths);

  print_array(z);

  // Now that we have the z-array, we can simply check if
  // a substring is periodic by watching it repeat throughout
  // the string.
  for (auto i: lengths) {
    int p = 0;
    bool found = true;
    while (p + i < str.size()) {
      p += i;

      // How much do we have left?
      int remaining = std::min(i, (int) (str.size() - p));

      // Do we see this repeat? If so we can keep going
      if (z[p] != remaining) {
        found = false;
        break;
      }
    }

    if (found) {
      std::cout << str.substr(0, i) << std::endl;
      break;
    }
  }

  // No repeating periods, print entire string
  std::cout << str << std::endl;

  return 0;
}
#endif

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