Problem p02730 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <iostream>
#include <list>
#include <set>
#include <string>
#include <unordered_map>
#include <vector>

#define rep(i, a, b) for (ll i = a; i < b; i++)

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> pll;
typedef unordered_map<string, ll> u_map;

// input
void input() {}
template <class Head, class... Tail> void input(Head &&a, Tail &&...b) {
  cin >> a;
  input(forward<Tail>(b)...);
}

template <class V> void input(vector<V> &v) {
  for_each(v.begin(), v.end(), [](auto &i) { input(i); });
}

// output
template <class V> void output(vector<V> &v) {
  for_each(v.begin(), v.end(), [](auto &x) { cout << x << " "; });
  cout << endl;
}

// math functions
ll mpow(ll a, ll n, ll p) {
  ll x = 1;
  while (n > 0) {
    if (n & 1) {
      x = a * x % p;
    }
    a = a * a % p;
    n >>= 1;
  }

  return x;
}

ll mfact(ll n, ll p) {
  ll res = 1;
  for (ll i = 2; i <= n; i++) {
    res = res * i % p;
  }
  return res;
}

ll mcomb(ll n, ll r, ll p) {
  if (n == 0 && r == 0) {
    return 1;
  }
  if (n < r || n < 0 || r < 0) {
    return 0;
  }

  ll a = mfact(n, p);
  ll b = mfact(n - r, p);
  ll c = mfact(r, p);
  return (((a * mpow(b, p - 2, p)) % p) * mpow(c, p - 2, p)) % p;
}

ll gcd(ll a, ll b) {
  if (a < b) {
    a ^= b;
    b ^= a;
    a ^= b;
  }
  return b ? gcd(b, a % b) : a;
}

ll lcm(ll a, ll b) { return a * b / gcd(a, b); }

bool isPallindrome(string s) {
  rep(i, 0, s.size() / 2) {
    if (s[i] != s[s.size() - i - 1]) {
      return false;
    }
  }
  return true;
}

int main() {
  string S;
  input(S);

  cout << ((isPallindrome(S) &&
            isPallindrome(S.substr(0, (S.size() - 1) / 2)) &&
            isPallindrome(S.substr((S.size() + 3) / 2 + 1)))
               ? "Yes"
               : "No")
       << endl;
  return 0;
}

Fixed submission:
#include <algorithm>
#include <iostream>
#include <list>
#include <set>
#include <string>
#include <unordered_map>
#include <vector>

#define rep(i, a, b) for (ll i = a; i < b; i++)

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> pll;
typedef unordered_map<string, ll> u_map;

// input
void input() {}
template <class Head, class... Tail> void input(Head &&a, Tail &&...b) {
  cin >> a;
  input(forward<Tail>(b)...);
}

template <class V> void input(vector<V> &v) {
  for_each(v.begin(), v.end(), [](auto &i) { input(i); });
}

// output
template <class V> void output(vector<V> &v) {
  for_each(v.begin(), v.end(), [](auto &x) { cout << x << " "; });
  cout << endl;
}

// math functions
ll mpow(ll a, ll n, ll p) {
  ll x = 1;
  while (n > 0) {
    if (n & 1) {
      x = a * x % p;
    }
    a = a * a % p;
    n >>= 1;
  }

  return x;
}

ll mfact(ll n, ll p) {
  ll res = 1;
  for (ll i = 2; i <= n; i++) {
    res = res * i % p;
  }
  return res;
}

ll mcomb(ll n, ll r, ll p) {
  if (n == 0 && r == 0) {
    return 1;
  }
  if (n < r || n < 0 || r < 0) {
    return 0;
  }

  ll a = mfact(n, p);
  ll b = mfact(n - r, p);
  ll c = mfact(r, p);
  return (((a * mpow(b, p - 2, p)) % p) * mpow(c, p - 2, p)) % p;
}

ll gcd(ll a, ll b) {
  if (a < b) {
    a ^= b;
    b ^= a;
    a ^= b;
  }
  return b ? gcd(b, a % b) : a;
}

ll lcm(ll a, ll b) { return a * b / gcd(a, b); }

bool isPallindrome(string s) {
  rep(i, 0, s.size() / 2) {
    if (s[i] != s[s.size() - i - 1]) {
      return false;
    }
  }
  return true;
}

int main() {
  string S;
  input(S);

  cout << ((isPallindrome(S) &&
            isPallindrome(S.substr(0, (S.size() - 1) / 2)) &&
            isPallindrome(S.substr((S.size() + 3) / 2 - 1)))
               ? "Yes"
               : "No")
       << endl;
  return 0;
}