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

#include <bits/stdc++.h>
#define rep(i, a) for (int i = (int)0; i < (int)a; ++i)
#define rrep(i, a) for (int i = (int)a - 1; i >= 0; --i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define RREP(i, a, b) for (int i = (int)a - 1; i >= b; --i)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
using ll = long long;
constexpr ll mod = 1e9 + 7;
constexpr ll INF = 1LL << 60;

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

ll gcd(ll n, ll m) {
  ll tmp;
  while (m != 0) {
    tmp = n % m;
    n = m;
    m = tmp;
  }
  return n;
}

ll lcm(ll n, ll m) {
  return abs(n) / gcd(n, m) * abs(m); // gl=xy
}

using namespace std;

void solve() {
  ll n, k;
  cin >> n >> k;
  vector<ll> a(n);
  rep(i, n) cin >> a[i], a[i]--;
  ll sz = 1;
  set<int> st;
  int cur = 0;
  st.insert(cur);
  vector<int> cnt(n);
  ll x = 0;
  rep(i, n) {
    cur = a[cur];
    if (st.count(cur)) {
      sz = i + 1 - cnt[cur];
      x = cnt[cur];
      break;
    } else {
      if (k == i + 1) {
        cout << cur + 1;
        return;
      }
      st.insert(cur);
      cnt[cur] = i + 1;
    }
  }
  vector<int> v(sz);
  rep(i, sz) {
    v[i] = cur;
    cur = a[cur];
    // cerr << v[i] << endl;
  }
  k = (k - x % sz + sz) % sz;
  // cout << k << endl;
  cout << v[k] + 1;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(15);
  solve();
  return 0;
}

Error: WA

Fixed submission:

#include <bits/stdc++.h>
#define rep(i, a) for (int i = (int)0; i < (int)a; ++i)
#define rrep(i, a) for (int i = (int)a - 1; i >= 0; --i)
#define REP(i, a, b) for (int i = (int)a; i < (int)b; ++i)
#define RREP(i, a, b) for (int i = (int)a - 1; i >= b; --i)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
using ll = long long;
constexpr ll mod = 1e9 + 7;
constexpr ll INF = 1LL << 60;

template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

ll gcd(ll n, ll m) {
  ll tmp;
  while (m != 0) {
    tmp = n % m;
    n = m;
    m = tmp;
  }
  return n;
}

ll lcm(ll n, ll m) {
  return abs(n) / gcd(n, m) * abs(m); // gl=xy
}

using namespace std;

void solve() {
  ll n, k;
  cin >> n >> k;
  vector<ll> a(n);
  rep(i, n) cin >> a[i], a[i]--;
  int sz = 1;
  set<int> st;
  int cur = 0;
  st.insert(cur);
  vector<int> cnt(n);
  int x = 0;
  rep(i, n) {
    cur = a[cur];
    if (st.count(cur)) {
      sz = i + 1 - cnt[cur];
      x = cnt[cur];
      break;
    } else {
      st.insert(cur);
      cnt[cur] = i + 1;
    }
  }
  vector<int> v(sz);
  rep(i, sz) {
    v[i] = cur;
    cur = a[cur];
  }
  k = (k - x + 1000000 * sz) % sz;
  // cout << k << endl;
  cout << v[k] + 1;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(15);
  solve();
  return 0;
}