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

#include <bits/stdc++.h>
#include <cassert>
typedef long long int ll;
using namespace std;

// @@ !! LIM(debug)
// --> f:<< debug
// ---- inserted function << from util.cc
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}

template <typename T1, typename T2, typename T3>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3> &t) {
  os << "(" << get<0>(t) << ", " << get<1>(t) << ", " << get<2>(t) << ")";
  return os;
}

template <typename T1, typename T2, typename T3, typename T4>
ostream &operator<<(ostream &os, const tuple<T1, T2, T3, T4> &t) {
  os << "(" << get<0>(t) << ", " << get<1>(t) << ", " << get<2>(t) << ", "
     << get<3>(t) << ")";
  return os;
}

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << '[';
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  os << ']';

  return os;
}

template <typename T, typename C>
ostream &operator<<(ostream &os, const set<T, C> &v) {
  os << '{';
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  os << '}';

  return os;
}

template <typename T, typename C>
ostream &operator<<(ostream &os, const unordered_set<T, C> &v) {
  os << '{';
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  os << '}';

  return os;
}

template <typename T, typename C>
ostream &operator<<(ostream &os, const multiset<T, C> &v) {
  os << '{';
  for (auto it = v.begin(); it != v.end(); it++) {
    if (it != v.begin())
      os << ", ";
    os << *it;
  }
  os << '}';

  return os;
}

template <typename T1, typename T2, typename C>
ostream &operator<<(ostream &os, const map<T1, T2, C> &mp) {
  os << '[';
  for (auto it = mp.begin(); it != mp.end(); it++) {
    if (it != mp.begin())
      os << ", ";
    os << it->first << ": " << it->second;
  }
  os << ']';

  return os;
}

template <typename T1, typename T2, typename C>
ostream &operator<<(ostream &os, const unordered_map<T1, T2, C> &mp) {
  os << '[';
  for (auto it = mp.begin(); it != mp.end(); it++) {
    if (it != mp.begin())
      os << ", ";
    os << it->first << ": " << it->second;
  }
  os << ']';

  return os;
}

template <typename T, typename T2>
ostream &operator<<(ostream &os, const queue<T, T2> &orig) {
  queue<T, T2> que(orig);
  bool first = true;
  os << '[';
  while (!que.empty()) {
    T x = que.front();
    que.pop();
    if (!first)
      os << ", ";
    os << x;
    first = false;
  }
  return os << ']';
}

template <typename T, typename T2, typename T3>
ostream &operator<<(ostream &os, const priority_queue<T, T2, T3> &orig) {
  priority_queue<T, T2, T3> pq(orig);
  bool first = true;
  os << '[';
  while (!pq.empty()) {
    T x = pq.top();
    pq.pop();
    if (!first)
      os << ", ";
    os << x;
    first = false;
  }
  return os << ']';
}

template <typename T> ostream &operator<<(ostream &os, const stack<T> &st) {
  stack<T> tmp(st);
  os << '[';
  bool first = true;
  while (!tmp.empty()) {
    T &t = tmp.top();
    if (first)
      first = false;
    else
      os << ", ";
    os << t;
    tmp.pop();
  }
  os << ']';
  return os;
}

#if __cplusplus >= 201703L
template <typename T> ostream &operator<<(ostream &os, const optional<T> &t) {
  if (t.has_value())
    os << "v(" << t.value() << ")";
  else
    os << "nullopt";
  return os;
}
#endif

ostream &operator<<(ostream &os, int8_t x) {
  os << (int32_t)x;
  return os;
}

// ---- end <<
// ---- inserted library file debug.cc
template <class... Args> string dbgFormat(const char *fmt, Args... args) {
  size_t len = snprintf(nullptr, 0, fmt, args...);
  char buf[len + 1];
  snprintf(buf, len + 1, fmt, args...);
  return string(buf);
}

template <class Head> void dbgLog(Head &&head) { cerr << head << endl; }

template <class Head, class... Tail> void dbgLog(Head &&head, Tail &&...tail) {
  cerr << head << " ";
  dbgLog(forward<Tail>(tail)...);
}

#if DEBUG
#define DLOG(...) dbgLog(__VA_ARGS__)
#define DFMT(...) cerr << dbgFormat(__VA_ARGS__) << endl
#define DCALL(func, ...) func(__VA_ARGS__)
#else
#define DLOG(...)
#define DFMT(...)
#define DCALL(func, ...)
#endif

#if DEBUG_LIB
#define DLOG_LIB(...) dbgLog(__VA_ARGS__)
#define DFMT_LIB(...) cerr << dbgFormat(__VA_ARGS__) << endl
#define DCALL_LIB(func, ...) func(__VA_ARGS__)
#else
#define DLOG_LIB(...)
#define DFMT_LIB(...)
#define DCALL_LIB(func, ...)
#endif

// ---- end debug.cc
// @@ !! LIM  -- end mark --

int main(/* int argc, char *argv[] */) {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << setprecision(20);

  ll N, K;
  cin >> N >> K;
  vector<ll> A(N + 1);
  for (ll i = 1; i <= N; i++)
    cin >> A[i];

  auto sim = [&](ll start, ll num) -> ll {
    ll cur = start;
    for (ll i = 0; i < num; i++)
      cur = A[cur];
    return cur;
  };

  auto detCycle = [&]() -> tuple<ll, ll, ll> {
    vector<ll> rec(N + 1, -1);
    rec[1] = 0;
    ll cur = 1;
    ll i = 1;
    while (true) {
      ll newC = A[cur];
      ll oldT = rec[newC];
      if (oldT >= 0)
        return {newC, oldT, i - oldT};
      rec[newC] = i;
      cur = newC;
      i++;
    }
  };

  ll goal;
  if (K <= N) {
    goal = sim(1, K);
  } else {
    auto [bp, taillen, cyclen] = detCycle();
    ll rem = K - taillen;
    if (rem < 0)
      goal = sim(1, K);
    else
      goal = sim(bp, rem % cyclen);
  }
  cout << goal << endl;

  return 0;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>
#include <cassert>
typedef long long int ll;
using namespace std;

// @@ !! LIM()

int main(/* int argc, char *argv[] */) {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << setprecision(20);

  ll N, K;
  cin >> N >> K;
  vector<ll> A(N + 1);
  for (ll i = 1; i <= N; i++)
    cin >> A[i];
  ll cur = 1;
  vector<ll> rec(N + 1, -1);
  rec[1] = 0;
  ll i = 1;
  bool checked = false;
  while (i <= K) {
    ll t = A[cur];
    if (checked || t < 0) {
      rec[t] = i;
      i++;
    } else {
      ll z = (K - i) / (i - t);
      i += z * (i - t);
      checked = true;
    }
    cur = t;
  }
  cout << cur << endl;

  return 0;
}