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

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

// #define int long long
#define rep(i, n) for (long long i = (long long)(0); i < (long long)(n); ++i)
#define reps(i, n) for (long long i = (long long)(1); i <= (long long)(n); ++i)
#define rrep(i, n) for (long long i = ((long long)(n)-1); i >= 0; i--)
#define rreps(i, n) for (long long i = ((long long)(n)); i > 0; i--)
#define irep(i, m, n)                                                          \\
  for (long long i = (long long)(m); i < (long long)(n); ++i)
#define ireps(i, m, n)                                                         \\
  for (long long i = (long long)(m); i <= (long long)(n); ++i)
#define SORT(v, n) sort(v, v + n);
#define REVERSE(v, n) reverse(v, v + n);
#define vsort(v) sort(v.begin(), v.end());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cout(d) cout << d << endl;
#define coutd(d) cout << std::setprecision(10) << d << endl;
#define cinline(n) getline(cin, n);
#define replace_all(s, b, a) replace(s.begin(), s.end(), b, a);
#define PI (acos(-1))
#define FILL(v, n, x) fill(v, v + n, x);
#define sz(x) long long(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vs = vector<string>;
using vpll = vector<pair<ll, ll>>;
using vtp = vector<tuple<ll, ll, ll>>;
using vb = vector<bool>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

const ll INF = 1e9;
const ll MOD = 1e9 + 7;
const ll LINF = 1e18;

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, k;
  cin >> n >> k;
  vll a(n);
  rep(i, n) cin >> a[i];

  vll s;
  vll ord(n + 1, -1);
  ll c = 1, l = 0;
  {
    ll v = 1;
    while (ord[v] == -1) {
      ord[v] = s.size();
      s.push_back(v);
      v = a[v - 1];
    }
    c = s.size() - ord[v];
    l = ord[v];
  }

  if (k < l)
    cout << s[k] << endl;
  else {
    k -= l;
    k %= c;
    cout << s[l + k] << endl;
  }
}

Error: 0

Fixed submission:

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

// #define int long long
#define rep(i, n) for (long long i = (long long)(0); i < (long long)(n); ++i)
#define reps(i, n) for (long long i = (long long)(1); i <= (long long)(n); ++i)
#define rrep(i, n) for (long long i = ((long long)(n)-1); i >= 0; i--)
#define rreps(i, n) for (long long i = ((long long)(n)); i > 0; i--)
#define irep(i, m, n)                                                          \\
  for (long long i = (long long)(m); i < (long long)(n); ++i)
#define ireps(i, m, n)                                                         \\
  for (long long i = (long long)(m); i <= (long long)(n); ++i)
#define SORT(v, n) sort(v, v + n);
#define REVERSE(v, n) reverse(v, v + n);
#define vsort(v) sort(v.begin(), v.end());
#define all(v) v.begin(), v.end()
#define mp(n, m) make_pair(n, m);
#define cout(d) cout << d << endl;
#define coutd(d) cout << std::setprecision(10) << d << endl;
#define cinline(n) getline(cin, n);
#define replace_all(s, b, a) replace(s.begin(), s.end(), b, a);
#define PI (acos(-1))
#define FILL(v, n, x) fill(v, v + n, x);
#define sz(x) long long(x.size())

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vs = vector<string>;
using vpll = vector<pair<ll, ll>>;
using vtp = vector<tuple<ll, ll, ll>>;
using vb = vector<bool>;

template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

const ll INF = 1e9;
const ll MOD = 1e9 + 7;
const ll LINF = 1e18;

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ll n, k;
  cin >> n >> k;
  vll a(n);
  rep(i, n) cin >> a[i], a[i]--;

  set<ll> s;
  vll b;
  ll now = 0;
  while (true) {
    if (s.count(now))
      break;
    b.push_back(now);
    s.insert(now);
    now = a[now];
  }

  ll m = b.size();
  // ll head=;
  ll hidx = a[b[m - 1]];
  // rep(i,m){
  //   if(b[i]==head) hidx=i;
  // }
  ll cycle = m - hidx;
  // cout<<cycle<<endl;

  if (k < m) {
    cout << b[k] << endl;
    return 0;
  }

  ll mod = (k - hidx) % cycle;
  ll ans = b[hidx + mod] + 1;
  cout << ans << endl;
}