Problem p02684 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
const int inf = 1e9 + 7;
const ll INF = 1e18;
const int mod = 1000000007;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
ll modpow(ll a, int b, int _mod) {
  return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (auto &vi : vec)
    os << vi << " ";
  return os;
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << p.F << " " << p.S;
  return os;
}
template <typename T> inline istream &operator>>(istream &is, vector<T> &v) {
  rep(j, sz(v)) is >> v[j];
  return is;
}
template <class T> inline void add(T &a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
}

void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);

  int T;
  // cin >> T;
  T = 1;

  while (T--) {
    solve();
  }
}

// https://atcoder.jp/contests/abc167/submissions/13082002
// 間違いの検証
void solve() {
  int n;
  cin >> n;
  string k;
  cin >> k;

  const int N = 200005;
  vector<vector<int>> G(N);
  vector<int> seen(N);
  int pos = -1;    // cycle 中に含まれる頂点 pos
  stack<int> hist; // 訪問履歴

  auto dfs = [&](auto &&dfs, int v, int p) -> void {
    seen[v] = 1;
    hist.push(v);
    for (auto nv : G[v]) {
      if (nv == p)
        continue; // ---------- 問題点を探す ----------
      if (seen[nv] == 2)
        continue;

      if (seen[nv] == 1) { // cycle を検出
        pos = nv;
        return;
      }
      dfs(dfs, nv, v);
      if (pos != -1)
        return;
    }
    hist.pop();
    seen[v] = 2;
  };

  rep(i, n) {
    int b;
    cin >> b;
    b--;
    G[i].eb(b);
  }

  ll x_ = stoll(k);
  if (x_ <= 10000000) { // ループに入る前に終わる
    int now = 0;
    rep(i, x_) { now = G[now][0]; }
    cout << now + 1 << endl;
    return;
  }

  dfs(dfs, 0, -1);

  // cycle 復元
  vector<int> cycle;
  while (!hist.empty()) {
    int t = hist.top();
    cycle.eb(t);
    hist.pop();
    if (t == pos)
      break;
  }
  reverse(all(cycle));
  // for (auto e : cycle) cout << e << " "; cout << endl;

  // ---------- 間違いはここか? ----------
  if (sz(cycle) == 0) {
    while (1) {
    }
  }

  int cnt = 0, now = 0;
  while (1) {
    if (now == cycle[0]) {
      break;
    }
    now = G[now][0];
    cnt++;
  }
  int k3 = 0;
  rep(i, sz(k)) {
    k3 *= 10;
    k3 += k[i] - '0';
    k3 %= sz(cycle);
  }
  k3 -= cnt; // 先に足すと, 10 倍するのが効いてしまう
  while (k3 < 0)
    k3 += sz(cycle);
  k3 %= sz(cycle);

  now = cycle[0];
  while (k3--) {
    now = G[now][0];
  }
  cout << now + 1 << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep3(i, l, n) for (int i = l; i < (n); ++i)
#define sz(v) (int)v.size()
const int inf = 1e9 + 7;
const ll INF = 1e18;
const int mod = 1000000007;
#define abs(x) (x >= 0 ? x : -(x))
#define lb(v, x) (int)(lower_bound(all(v), x) - v.begin())
#define ub(v, x) (int)(upper_bound(all(v), x) - v.begin())
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> T gcd(T a, T b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template <typename T> T pow(T a, int b) {
  return b ? pow(a * a, b / 2) * (b % 2 ? a : 1) : 1;
}
ll modpow(ll a, int b, int _mod) {
  return b ? modpow(a * a % _mod, b / 2, _mod) * (b % 2 ? a : 1) % _mod : 1;
}
template <class T> ostream &operator<<(ostream &os, const vector<T> &vec) {
  for (auto &vi : vec)
    os << vi << " ";
  return os;
}
template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << p.F << " " << p.S;
  return os;
}
template <typename T> inline istream &operator>>(istream &is, vector<T> &v) {
  rep(j, sz(v)) is >> v[j];
  return is;
}
template <class T> inline void add(T &a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
}

void solve();

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(10);

  int T;
  // cin >> T;
  T = 1;

  while (T--) {
    solve();
  }
}

// https://atcoder.jp/contests/abc167/submissions/13082002
// 間違いの検証
void solve() {
  int n;
  cin >> n;
  string k;
  cin >> k;

  const int N = 200005;
  vector<vector<int>> G(N);
  vector<int> seen(N);
  int pos = -1;    // cycle 中に含まれる頂点 pos
  stack<int> hist; // 訪問履歴

  auto dfs = [&](auto &&dfs, int v, int p) -> void {
    seen[v] = 1;
    hist.push(v);
    for (auto nv : G[v]) {
      if (nv == p)
        continue; // ---------- 問題点を探す ----------
      if (seen[nv] == 2)
        continue;

      if (seen[nv] == 1) { // cycle を検出
        pos = nv;
        return;
      }
      dfs(dfs, nv, v);
      if (pos != -1)
        return;
    }
    hist.pop();
    seen[v] = 2;
  };

  rep(i, n) {
    int b;
    cin >> b;
    b--;
    G[i].eb(b);
  }

  ll x_ = stoll(k);
  if (x_ <= 10000000) { // ループに入る前に終わる
    int now = 0;
    rep(i, x_) { now = G[now][0]; }
    cout << now + 1 << endl;
    return;
  }

  dfs(dfs, 0, -1);

  // cycle 復元
  vector<int> cycle;
  while (!hist.empty()) {
    int t = hist.top();
    cycle.eb(t);
    hist.pop();
    if (t == pos)
      break;
  }
  reverse(all(cycle));
  // for (auto e : cycle) cout << e << " "; cout << endl;

  // 本当は cycle の size が 2
  if (sz(cycle) == 0) { // ---------- あってる? ----------
    ll k_ = stoll(k);
    vector<int> v(n + 5);
    v[1] = G[0][0];
    rep3(i, 2, n + 5) v[i] = G[v[i - 1]][0];
    // cout << "v " << v << endl;
    rep(i, n + 5) {
      if (v[i] == v[i + 2]) {
        k_ %= 2;
        (k_ = k_ - i % 2 + 2) %= 2;
        if (k_ % 2 == 0)
          cout << v[i] + 1 << endl;
        else
          cout << v[i + 1] + 1 << endl;
        return;
      }
    }
  }

  int cnt = 0, now = 0;
  while (1) {
    if (now == cycle[0]) {
      break;
    }
    now = G[now][0];
    cnt++;
  }
  int k3 = 0;
  rep(i, sz(k)) {
    k3 *= 10;
    k3 += k[i] - '0';
    k3 %= sz(cycle);
  }
  k3 -= cnt; // 先に足すと, 10 倍するのが効いてしまう
  while (k3 < 0)
    k3 += sz(cycle);
  k3 %= sz(cycle);

  now = cycle[0];
  while (k3--) {
    now = G[now][0];
  }
  cout << now + 1 << endl;
}