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

#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <bitset>    // bitset
#include <cctype>    // isupper, islower, isdigit, toupper, tolower
#include <cmath>
#include <cstdint>  // int64_t, int*_t
#include <cstdio>   // printf
#include <deque>    // deque
#include <iostream> // cout, endl, cin
#include <map>      // map
#include <math.h>
#include <queue>         // queue, priority_queue
#include <set>           // set
#include <stack>         // stack
#include <string>        // string, to_string, stoi
#include <tuple>         // tuple, make_tuple
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <utility>       // pair, make_pair
#include <vector>        // vector
using namespace std;

#define int long long
#define endl "\\n"
#define all(v) v.begin(), v.end()
#define fir first
#define sec second
#define m_p make_pair
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define vecin(v)                                                               \\
  for (int i = 0; i < (int)v.size(); i++)                                      \\
    cin >> v[i];
#define vecout(v)                                                              \\
  for (int i = 0; i < v.size(); i++)                                           \\
    cout << v[i] << " ";                                                       \\
  cout << endl;

#define LLU long long unsigned

const int mod = 1000000007;
const int INF = 2000000015;
const int MAX = 510000;
const int MOD = 1000000007;
long long fac[MAX], finv[MAX], inv[MAX];

int jousu(int x00, int y00) {
  int z00 = 1;
  for (int i = 0; i < y00; i++) {
    z00 *= x00;
  }
  return z00;
}
int keta(int x00) {
  int z00 = x00;
  int w00 = 0;
  while (z00 != 0) {
    z00 /= 10;
    w00++;
  }
  return w00;
}
int krag(double xx) { return INF - (int)(INF - xx); }
// x未満の要素数を返す二分探索関数
int b_s(vector<int> &vecxx, int xx) {
  if (xx <= vecxx[0]) { // x以下の要素数を返させる時は"<"に変える
    return 0;
  }
  if (vecxx.back() < xx) { // x以下の要素数を返させる時は"<="に変える
    return vecxx.size();
  }
  int aa1 = 0, aa2 = vecxx.size() - 1, aa3;
  while (aa1 + 1 < aa2) {
    aa3 = (aa1 + aa2) / 2;
    if (vecxx[aa3] >= xx) { // x以下の要素数を返させる時は">"に変える
      aa2 = aa3;
    } else {
      aa1 = aa3;
    }
  }
  return aa2;
}

// テーブルを作る前処理
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
// n個を1個以上のx組のグループに分ける重複組み合わせはcom(n-1,x-1)
// グループが0個でもいいときはnにxを足す
//  二項係数計算
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
int modinv(int a, int m) {
  int b = m, u = 1, v = 0;
  while (b) {
    int t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}
int modpow(int a, int n) {
  int res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}

int gcd(int xx, int yy) {
  int p = xx;
  int q = yy;
  if (q > p)
    swap(p, q);
  while (p % q != 0) {
    p %= q;
    swap(p, q);
  }
  return q;
}
int lcm(int xx, int yy) { return xx * yy / gcd(xx, yy); }
bool prime(int xx) {
  int a = xx;
  for (int i = 2; i * i <= xx; i++) {
    if (xx % i == 0) {
      return 0;
    }
  }
  return 1;
}

signed main() {
  int n, k, a = 0, b = 0, c;
  cin >> n >> k;
  vector<int> vec(n), vec2;
  set<int> se;
  rep(i, n) {
    cin >> c;
    vec[i] = c - 1;
  }
  while (!se.count(a)) {
    vec2.push_back(a);
    se.insert(a);
    a = vec[a];
  }
  while (vec2[b] != a) {
    b++;
  }
  if (vec2.size() - 1 > k) {
    cout << vec2[k] + 1 << endl;
    return 0;
  }
  cout << vec2[b + ((k - b) % (vec2.size() - b))] + 1 << endl;
}


Error: -8

Fixed submission:

#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <bitset>    // bitset
#include <cctype>    // isupper, islower, isdigit, toupper, tolower
#include <cmath>
#include <cstdint>  // int64_t, int*_t
#include <cstdio>   // printf
#include <deque>    // deque
#include <iostream> // cout, endl, cin
#include <map>      // map
#include <math.h>
#include <queue>         // queue, priority_queue
#include <set>           // set
#include <stack>         // stack
#include <string>        // string, to_string, stoi
#include <tuple>         // tuple, make_tuple
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <utility>       // pair, make_pair
#include <vector>        // vector
using namespace std;

#define int long long
#define endl "\\n"
#define all(v) v.begin(), v.end()
#define fir first
#define sec second
#define m_p make_pair
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define vecin(v)                                                               \\
  for (int i = 0; i < (int)v.size(); i++)                                      \\
    cin >> v[i];
#define vecout(v, n)                                                           \\
  for (int i = 0; i < n; i++)                                                  \\
    cout << v[i] << " ";                                                       \\
  cout << endl;

const int mod = 1000000007;
const int INF = 2000000015;
const int MAX = 510000;
const int MOD = 1000000007;
long long fac[MAX], finv[MAX], inv[MAX];
int jousu(int x00, int y00) {
  int z00 = 1;
  for (int i = 0; i < y00; i++) {
    z00 *= x00;
  }
  return z00;
}
int keta(int x00) {
  int z00 = x00;
  int w00 = 0;
  while (z00 != 0) {
    z00 /= 10;
    w00++;
  }
  return w00;
}
int krag(double xx) { return INF - (int)(INF - xx); }

// x未満の要素数を返す二分探索関数
int b_s(vector<int> &vecxx, int xx) {
  if (xx <= vecxx[0]) { // x以下の要素数を返させる時は"<"に変える
    return 0;
  }
  if (vecxx.back() < xx) { // x以下の要素数を返させる時は"<="に変える
    return vecxx.size();
  }
  int aa1 = 0, aa2 = vecxx.size() - 1, aa3;
  while (aa1 + 1 < aa2) {
    aa3 = (aa1 + aa2) / 2;
    if (vecxx[aa3] >= xx) { // x以下の要素数を返させる時は">"に変える
      aa2 = aa3;
    } else {
      aa1 = aa3;
    }
  }
  return aa2;
}

int modinv(int a, int m) {
  int b = m, u = 1, v = 0;
  while (b) {
    int t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}
int modpow(int a, int n) {
  int res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
// n個を1個以上のx組のグループに分ける重複組み合わせはcom(n-1,x-1)
// グループが0個でもいいときはnにxを足す

int com(int xx, int yy) {
  int zz = 1;
  for (int i = xx; i > xx - yy; i--) {
    zz *= i;
    zz %= mod;
  }
  for (int i = 1; i <= yy; i++) {
    zz *= modinv(i, mod);
    zz %= mod;
  }
  return zz;
}
int gcd(int xx, int yy) {
  int p = xx;
  int q = yy;
  if (q > p)
    swap(p, q);
  while (p % q != 0) {
    p %= q;
    swap(p, q);
  }
  return q;
}
int lcm(int xx, int yy) { return xx * yy / gcd(xx, yy); }
bool prime(int xx) {
  int a = xx;
  for (int i = 2; i * i <= xx; i++) {
    if (xx % i == 0) {
      return 0;
    }
  }
  return 1;
}

signed main() {
  int n, k, a = 0, b = 1;
  cin >> n >> k;
  vector<int> vec(n), vec2;
  vec2.push_back(0);
  set<int> se;
  se.insert(0);
  rep(i, n) {
    cin >> a;
    vec[i] = a - 1;
  }
  a = vec[0];
  while (!se.count(a)) {
    vec2.push_back(a);
    se.insert(a);
    a = vec[a];
    b++;
  }
  int j = 0;
  while (a != vec[j]) {
    j++;
  }
  b -= j;
  cout << vec2[((k - j) % b) + j] + 1 << endl;
}