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

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using ll = long long;
using vi = vector<ll>;
template <class T> void out(T a) { cout << a << '\\n'; }
template <class T> void outv(T v) {
  rep(i, v.size()) {
    if (i)
      cout << ' ';
    cout << v[i];
  }
  cout << '\\n';
}
int main() {
  ll n, k;
  cin >> n >> k;
  vi a(n);
  rep(i, n) cin >> a[i];
  rep(i, n) a[i]--;
  ll now = 0, p, q;
  vi b(n, -1); // 訪問
  rep(i, n) {
    b[now] = i;
    now = a[now];
    if (b[now] > -1) {
      p = b[now];
      q = i - p + 1;
      break;
    }
  }
  ll x;
  if (k < p)
    x = k;
  else
    x = (k - p) % q + p;
  rep(i, n) if (b[i] == x) out(i + 1);
}

Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
// #pragma GCC optimize("Ofast")
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define REP(i, k, n) for (long long i = k; i < (long long)(n); i++)
#define all(a) a.begin(), a.end()
#define eb emplace_back
#define lb(v, k) (lower_bound(all(v), k) - v.begin())
#define ub(v, k) (upper_bound(all(v), k) - v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T, vector<T>, greater<T>>
using ll = long long;
using P = pair<ll, ll>;
using PP = tuple<ll, ll, ll>;
using S = multiset<ll>;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
const ll inf = 1001001001001001;
const int INF = 1001001001;
const int mod = 1000000007;
const double eps = 1e-10;
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> void out(T a) { cout << a << '\\n'; }
template <class T> void outp(T a) {
  cout << '(' << a.fi << ',' << a.se << ')' << '\\n';
}
template <class T> void outvp(T v) {
  rep(i, v.size()) cout << '(' << v[i].fi << ',' << v[i].se << ')';
  cout << '\\n';
}
template <class T> void outv(T v) {
  rep(i, v.size()) {
    if (i)
      cout << ' ';
    cout << v[i];
  }
  cout << '\\n';
}
template <class T> void outvv(T v) { rep(i, v.size()) outv(v[i]); }
template <class T> bool isin(T x, T l, T r) { return (l) <= (x) && (x) <= (r); }
template <class T> void YesNo(T b) {
  if (b)
    out("Yes");
  else
    out("No");
}
void decimal(int x) { cout << fixed << setprecision(x); }
// ll GCD(ll a,ll b){if(b==0) return a; return GCD(b,a%b);}
// ll LCM(ll a,ll b){return(a/GCD(a,b))*(b/GCD(a,b))*GCD(a,b);}
ll POW(ll a, ll b) {
  a %= mod;
  if (b == 0)
    return 1;
  if (b & 1)
    return a * POW(a, b - 1) % mod;
  ll k = POW(a, b / 2);
  return k * k % mod;
}
vi calc(ll x) {
  vi res;
  while (x > 0) {
    res.eb(x % 10);
    x /= 10;
  }
  reverse(all(res));
  return res;
}

void solve() {
  ll n, k;
  cin >> n >> k;
  vi a(n);
  rep(i, n) cin >> a[i];
  ll now = 1, cnt = 0;
  vi b = {1};
  vector<bool> vb(n, false);
  while (true) {
    vb[now - 1] = true;
    now = a[now - 1];
    cnt++;
    if (vb[now - 1])
      break;
    b.eb(now);
  }
  vi v;
  bool f = false;
  ll yobn;
  rep(i, cnt) {
    if (b[i] == now) {
      yobn = i;
      f = true;
    }
    if (f)
      v.eb(b[i]);
  }
  out(v[(k - yobn) % (ll)(v.size())]);
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}