Problem p02684 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
#define fast_io                                                                \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define ll long long int
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = n - 1; i >= 0; i--)
#define REPS(i, n) for (int i = n; i > 0; i--)
#define MOD (long long int)(1e9 + 7)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a) < (b)) ? (b) : (a))
#define chmin(a, b) a = (((a) > (b)) ? (b) : (a))
#define all(v) v.begin(), v.end()
typedef pair<int, int> Pii;
typedef pair<ll, ll> Pll;

ll mpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = mpow(a, b / 2);
    return memo * memo % MOD;
  } else
    return mpow(a, b - 1) * a % MOD;
}
ll lpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n) {
  if (kaijo_memo.size() > n)
    return kaijo_memo[n];
  if (kaijo_memo.size() == 0)
    kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n) {
  if (gyaku_kaijo_memo.size() > n)
    return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0)
    gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
                               mpow(gyaku_kaijo_memo.size(), MOD - 2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r) {
  if (n == r)
    return 1; // 0個の丸と-1個の棒みたいな時に時に効く？不安.
  if (n < r || r < 0)
    return 0;
  ll ret = 1;
  if (n <= 1e7) {
    ret *= kaijo(n);
    ret %= MOD;
    ret *= gyaku_kaijo(r);
    ret %= MOD;
    ret *= gyaku_kaijo(n - r);
    ret %= MOD;
  } else {
    rep(i, r) {
      ret *= n - i;
      ret %= MOD;
      ret *= mpow(r - i, MOD - 2);
      ret %= MOD;
    }
  }
  return ret;
}

int main() {
  ll n, k;
  cin >> n >> k;
  vector<int> a(n);
  rep(i, n) {
    cin >> a[i];
    a[i]--;
  }
  int c = 0;
  ll cnt = 1;
  ll ant = 0;
  vector<int> memo(n, -1);
  memo[0] = 0;
  while (1) {
    c = a[c];
    // cout << c <<endl;
    int u = 1;
    if (memo[c] != -1) {
      u = 0;
      ant = cnt - memo[c];
    }
    if (u == 0) {
      break;
    }
    memo[c] = cnt;
    cnt++;
  }
  int x = 0;
  if (k < cnt - ant) {
    rep(i, k) { x = a[x]; }
  } else {
    k = k - (cnt - ant);
    // k = k-cnt+ant;
    k = k % ant;
    rep(i, k + cnt - ant) { x = a[x]; }
  }
  // cout << ant << " " << cnt << endl;
  cout << x + 1 << endl;
  return 0;
}


Error: 0

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
#define fast_io                                                                \\
  ios_base::sync_with_stdio(false);                                            \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
#define ll long long int
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define reps(i, n) for (int i = 1; i <= (int)(n); i++)
#define REP(i, n) for (int i = n - 1; i >= 0; i--)
#define REPS(i, n) for (int i = n; i > 0; i--)
#define MOD (long long int)(1e9 + 7)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a) < (b)) ? (b) : (a))
#define chmin(a, b) a = (((a) > (b)) ? (b) : (a))
#define all(v) v.begin(), v.end()
typedef pair<int, int> Pii;
typedef pair<ll, ll> Pll;

ll mpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = mpow(a, b / 2);
    return memo * memo % MOD;
  } else
    return mpow(a, b - 1) * a % MOD;
}
ll lpow(ll a, ll b) {
  if (b == 0)
    return 1;
  else if (b % 2 == 0) {
    ll memo = lpow(a, b / 2);
    return memo * memo;
  } else
    return lpow(a, b - 1) * a;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n) {
  if (kaijo_memo.size() > n)
    return kaijo_memo[n];
  if (kaijo_memo.size() == 0)
    kaijo_memo.push_back(1);
  while (kaijo_memo.size() <= n)
    kaijo_memo.push_back(kaijo_memo[kaijo_memo.size() - 1] * kaijo_memo.size() %
                         MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n) {
  if (gyaku_kaijo_memo.size() > n)
    return gyaku_kaijo_memo[n];
  if (gyaku_kaijo_memo.size() == 0)
    gyaku_kaijo_memo.push_back(1);
  while (gyaku_kaijo_memo.size() <= n)
    gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size() - 1] *
                               mpow(gyaku_kaijo_memo.size(), MOD - 2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r) {
  if (n == r)
    return 1; // 0個の丸と-1個の棒みたいな時に時に効く？不安.
  if (n < r || r < 0)
    return 0;
  ll ret = 1;
  if (n <= 1e7) {
    ret *= kaijo(n);
    ret %= MOD;
    ret *= gyaku_kaijo(r);
    ret %= MOD;
    ret *= gyaku_kaijo(n - r);
    ret %= MOD;
  } else {
    rep(i, r) {
      ret *= n - i;
      ret %= MOD;
      ret *= mpow(r - i, MOD - 2);
      ret %= MOD;
    }
  }
  return ret;
}

int main() {
  ll n, k;
  cin >> n >> k;
  vector<int> a(n);
  rep(i, n) {
    cin >> a[i];
    a[i]--;
  }
  int c = 0;
  ll cnt = 1;
  ll ant = 0;
  vector<int> memo(n, -1);
  memo[0] = 0;
  while (1) {
    c = a[c];
    // cout << c <<endl;
    int u = 1;
    if (memo[c] != -1) {
      u = 0;
      ant = cnt - memo[c];
    }
    if (u == 0) {
      break;
    }
    memo[c] = cnt;
    cnt++;
  }
  int x = 0;
  if (k < ant - cnt) {
    rep(i, k) { x = a[x]; }
  } else {
    assert(k - cnt + ant >= 0);
    k = k - cnt + ant;
    k = k % ant;
    rep(i, k + cnt - ant) { x = a[x]; }
  }
  // cout << ant << " " << cnt << endl;
  cout << x + 1 << endl;
  return 0;
}