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

#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
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 Per(i, sta, n) for (int i = n - 1; i >= sta; 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++)
typedef long double ld;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

int n;
ll k;
ll num = 0;
int a[200010];
bool visited[200010];
vector<int> cycle = {};
int ans = -1;

void dfs(int s) {
  // cout << s << " " << num << endl;
  if (visited[s]) {
    int now = s;
    while (cycle.empty() || now != s) {
      // cout << now << endl;
      cycle.push_back(now);
      now = a[now];
    }
    return;
  }
  visited[s] = true;
  num += 1;
  if (k == num)
    ans = a[s];
  dfs(a[s]);
}

void solve() {
  cin >> n >> k;
  rep(i, n) {
    cin >> a[i];
    a[i]--;
  }
  dfs(0);
  // for(int c:cycle){
  //     cout << c << endl;
  // }
  ll siz = cycle.size();
  if (ans == -1) {
    cout << cycle[(k - num) % siz] + 1 << endl;
  } else
    cout << ans + 1 << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}

Error: 0

Fixed submission:

#include <algorithm>
#include <bitset>
#include <ciso646>
#include <cmath>
#include <complex>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
typedef long long ll;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (ll)1000000007 * 1000000007;
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 Per(i, sta, n) for (int i = n - 1; i >= sta; 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++)
typedef long double ld;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<ll, ll> LP;
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

int n;
ll k;
ll num = 0;
int a[200010];
bool visited[200010];
vector<int> cycle = {};

void dfs(int s) {
  // cout << s << endl;
  if (visited[s]) {
    int now = s;
    while (cycle.empty() || now != s) {
      // cout << now << endl;
      cycle.push_back(now);
      now = a[now];
    }
    return;
  }
  visited[s] = true;
  num += 1;
  dfs(a[s]);
}

void solve() {
  cin >> n >> k;
  rep(i, n) {
    cin >> a[i];
    a[i]--;
  }
  dfs(0);
  // for(int c:cycle){
  //     cout << c << endl;
  // }
  ll siz = cycle.size();
  cout << cycle[(k - num) % siz] + 1 << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}