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

#include <bits/stdc++.h>
using namespace std;

// #define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = mod * mod;
const int INF_N = 1e+9;
typedef pair<int, int> P;
#define stop                                                                   \\
  char nyaa;                                                                   \\
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
#define all(v) (v).begin(), (v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-12;
const ld pi = acos(-1.0);
// typedef vector<vector<ll>> mat;
typedef vector<int> vec;

// 繰り返し二乗法
ll mod_pow(ll a, ll n, ll m) {
  ll res = 1;
  while (n) {
    if (n & 1)
      res = res * a % m;
    a = a * a % m;
    n >>= 1;
  }
  return res;
}

struct modint {
  ll n;
  modint() : n(0) { ; }
  modint(ll m) : n(m) {
    if (n >= mod)
      n %= mod;
    else if (n < 0)
      n = (n % mod + mod) % mod;
  }
  operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint &a, modint b) {
  a.n += b.n;
  if (a.n >= mod)
    a.n -= mod;
  return a;
}
modint operator-=(modint &a, modint b) {
  a.n -= b.n;
  if (a.n < 0)
    a.n += mod;
  return a;
}
modint operator*=(modint &a, modint b) {
  a.n = ((ll)a.n * b.n) % mod;
  return a;
}
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
  if (n == 0)
    return modint(1);
  modint res = (a * a) ^ (n / 2);
  if (n % 2)
    res = res * a;
  return res;
}

// 逆元(Eucledean algorithm)
ll inv(ll a, ll p) { return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p); }
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

const int max_n = 1 << 18;
modint fact[max_n], factinv[max_n];
void init_f() {
  fact[0] = modint(1);
  for (int i = 0; i < max_n - 1; i++) {
    fact[i + 1] = fact[i] * modint(i + 1);
  }
  factinv[max_n - 1] = modint(1) / fact[max_n - 1];
  for (int i = max_n - 2; i >= 0; i--) {
    factinv[i] = factinv[i + 1] * modint(i + 1);
  }
}
modint comb(int a, int b) {
  if (a < 0 || b < 0 || a < b)
    return 0;
  return fact[a] * factinv[b] * factinv[a - b];
}
using mP = pair<modint, modint>;

int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

ll N, K;
vec a;
vector<P> pv;
unordered_map<int, int> m;
int aa = 0, bb = 0;
int kk = 0;
void dfs(int n, int d) {
  if (d == K) {
    kk = n;
    return;
  }
  if (m[n] != 0) {
    aa = n;
    bb = d;
    pv.push_back(make_pair(n, d));
    kk = -1;
    return;
  }
  pv.push_back(make_pair(n, d));
  m[n] = d + 1;
  dfs(a[n], d + 1);
}

void solve() {
  cin >> N >> K;
  a.resize(N);
  int cc = 0;
  vec v;
  rep(i, N) {
    cin >> a[i];
    a[i]--;
  }
  dfs(0, 0);
  if (kk != -1) {
    cout << kk + 1 << endl;
  } else {
    cc = bb - (m[aa] - 1);
    v.resize(cc);
    rep(i, cc) { v[pv[bb - i].second % cc] = pv[bb - i].first; }
    cout << v[K % cc] + 1 << endl;
  }
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout << fixed << setprecision(10);
  // init_f();
  // init();
  // int t; cin >> t; rep(i, t)solve();
  solve();
  //   stop
  return 0;
}


Error: 0

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

// #define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = mod * mod;
const int INF_N = 1e+9;
typedef pair<int, int> P;
#define stop                                                                   \\
  char nyaa;                                                                   \\
  cin >> nyaa;
#define rep(i, n) for (int i = 0; i < n; i++)
#define per(i, n) for (int i = n - 1; i >= 0; i--)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define rep1(i, n) for (int i = 1; i <= n; i++)
#define per1(i, n) for (int i = n; i >= 1; i--)
#define Rep1(i, sta, n) for (int i = sta; i <= n; i++)
#define all(v) (v).begin(), (v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-12;
const ld pi = acos(-1.0);
// typedef vector<vector<ll>> mat;
typedef vector<int> vec;

// 繰り返し二乗法
ll mod_pow(ll a, ll n, ll m) {
  ll res = 1;
  while (n) {
    if (n & 1)
      res = res * a % m;
    a = a * a % m;
    n >>= 1;
  }
  return res;
}

struct modint {
  ll n;
  modint() : n(0) { ; }
  modint(ll m) : n(m) {
    if (n >= mod)
      n %= mod;
    else if (n < 0)
      n = (n % mod + mod) % mod;
  }
  operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint &a, modint b) {
  a.n += b.n;
  if (a.n >= mod)
    a.n -= mod;
  return a;
}
modint operator-=(modint &a, modint b) {
  a.n -= b.n;
  if (a.n < 0)
    a.n += mod;
  return a;
}
modint operator*=(modint &a, modint b) {
  a.n = ((ll)a.n * b.n) % mod;
  return a;
}
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
  if (n == 0)
    return modint(1);
  modint res = (a * a) ^ (n / 2);
  if (n % 2)
    res = res * a;
  return res;
}

// 逆元(Eucledean algorithm)
ll inv(ll a, ll p) { return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p); }
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

const int max_n = 1 << 18;
modint fact[max_n], factinv[max_n];
void init_f() {
  fact[0] = modint(1);
  for (int i = 0; i < max_n - 1; i++) {
    fact[i + 1] = fact[i] * modint(i + 1);
  }
  factinv[max_n - 1] = modint(1) / fact[max_n - 1];
  for (int i = max_n - 2; i >= 0; i--) {
    factinv[i] = factinv[i + 1] * modint(i + 1);
  }
}
modint comb(int a, int b) {
  if (a < 0 || b < 0 || a < b)
    return 0;
  return fact[a] * factinv[b] * factinv[a - b];
}
using mP = pair<modint, modint>;

int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

ll N, K;
vec a;
vector<P> pv;
unordered_map<int, int> m;
int aa = 0, bb = 0;
int dfs(int n, int d) {
  if (d == K)
    return n;
  if (m[n] != 0) {
    aa = n;
    bb = d;
    pv.push_back(make_pair(n, d));
    return -1;
  }
  pv.push_back(make_pair(n, d));
  m[n] = d + 1;
  dfs(a[n], d + 1);
  return -1;
}

void solve() {
  cin >> N >> K;
  a.resize(N);
  int cc = 0;
  vec v;
  rep(i, N) {
    cin >> a[i];
    a[i]--;
  }
  int tmp = dfs(0, 0);
  if (tmp != -1) {
    cout << tmp + 1 << endl;
  } else {
    cc = bb - (m[aa] - 1);
    v.resize(cc);
    rep(i, cc) { v[pv[bb - i].second % cc] = pv[bb - i].first; }
    cout << v[K % cc] + 1 << endl;
  }
}

signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout << fixed << setprecision(10);
  // init_f();
  // init();
  // int t; cin >> t; rep(i, t)solve();
  solve();
  //   stop
  return 0;
}