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

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
// #define P pair<ll,ll>
#define FOR(I, A, B) for (ll I = ll(A); I < ll(B); ++I)
#define FORR(I, A, B) for (ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x, t, f) ((x) ? (t) : (f))
#define SORT(x) (sort(x.begin(), x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x, v)                                                             \\
  (lower_bound(x.begin(), x.end(), v) - x.begin()) // xi>=v  x is sorted
#define POSU(x, v)                                                             \\
  (upper_bound(x.begin(), x.end(), v) - x.begin()) // xi>v  x is sorted
#define NUM(x, v) (POSU(x, v) - POSL(x, v))        // x is sorted
#define REV(x) (reverse(x.begin(), x.end()))       // reverse
ll gcd(ll a, ll b) {
  if (a % b == 0)
    return b;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) {
  ll c = gcd(a, b);
  return ((a / c) * (b / c) * c);
}
#define NEXTP(x) next_permutation(x.begin(), x.end())
const ll INF = ll(1e16) + ll(7);
const ll MOD = 1000000007LL;
#define out(a) cout << fixed << setprecision((a))
// tie(a,b,c) = make_tuple(10,9,87);
#define pop(a) __builtin_popcount((a))
ll keta(ll a) {
  ll r = 0;
  while (a) {
    a /= 10;
    r++;
  }
  return r;
}

int main() {

  ll N, k;
  cin >> N >> k;

  vector<ll> A(N), cnt(N, INF);
  FOR(i, 0, N) cin >> A[i];
  FOR(i, 0, N) A[i]--;

  cnt[0] = 0;
  ll now = 0;

  FOR(i, 0, 3 * N) {

    if (k == 0) {
      cout << now + 1 << endl;
      return 0;
    }
    ll nex = A[now];
    if (nex == now) {
      cout << now + 1 << endl;
      return 0;
    }

    if (cnt[nex] == INF)
      cnt[nex] = cnt[now] + 1;

    if (cnt[nex] < cnt[now]) {
      ll loop = abs(cnt[now] - cnt[nex]) + 1;
      k -= (k / loop) * loop;
    }

    if (k == 0) {
      cout << now + 1 << endl;
      return 0;
    }

    now = nex;
    k--;
  }
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
// #define P pair<ll,ll>
#define FOR(I, A, B) for (ll I = ll(A); I < ll(B); ++I)
#define FORR(I, A, B) for (ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x, t, f) ((x) ? (t) : (f))
#define SORT(x) (sort(x.begin(), x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x, v)                                                             \\
  (lower_bound(x.begin(), x.end(), v) - x.begin()) // xi>=v  x is sorted
#define POSU(x, v)                                                             \\
  (upper_bound(x.begin(), x.end(), v) - x.begin()) // xi>v  x is sorted
#define NUM(x, v) (POSU(x, v) - POSL(x, v))        // x is sorted
#define REV(x) (reverse(x.begin(), x.end()))       // reverse
ll gcd(ll a, ll b) {
  if (a % b == 0)
    return b;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) {
  ll c = gcd(a, b);
  return ((a / c) * (b / c) * c);
}
#define NEXTP(x) next_permutation(x.begin(), x.end())
const ll INF = ll(1e16) + ll(7);
const ll MOD = 1000000007LL;
#define out(a) cout << fixed << setprecision((a))
// tie(a,b,c) = make_tuple(10,9,87);
#define pop(a) __builtin_popcount((a))
ll keta(ll a) {
  ll r = 0;
  while (a) {
    a /= 10;
    r++;
  }
  return r;
}

int main() {

  ll N, k;
  cin >> N >> k;

  vector<ll> A(N), cnt(N, INF);
  FOR(i, 0, N) cin >> A[i];
  FOR(i, 0, N) A[i]--;

  cnt[0] = 0;
  ll now = 0;

  FOR(i, 0, 6 * N) {
    if (k == 0) {
      cout << now + 1 << endl;
      return 0;
    }
    ll nex = A[now];
    if (nex == now) {
      cout << now + 1 << endl;
      return 0;
    }
    if (cnt[now] == INF)
      cnt[now] = i + 1;
    if (cnt[nex] < INF) {
      ll loop = cnt[now] - cnt[nex] + 1;
      k %= loop;
    }
    if (k == 0) {
      cout << now + 1 << endl;
      return 0;
    }
    now = nex;
    k--;
  }
}