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

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
using vll = vector<ll>;
using vvll = vector<vll>;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vb = vector<bool>;
using pii = pair<int, int>;
using vpii = vector<pair<int, int>>;

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

// pow(llpow,modpow)
template <class T> ll llpow(ll x, T n) {
  ll ans = 1;
  if (x == 0)
    ans = 0;
  while (n) {
    if (n & 1)
      ans *= x;
    x *= x;
    n >>= 1;
  }
  return ans;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
// 最大公約数
template <class T> inline T gcd(T x, T y) {
  if (y == 0)
    return x;
  else {
    return gcd(y, x % y);
  }
}
// 最小公倍数
template <class T> inline T lcm(T x, T y) { return x / gcd(x, y) * y; }
// 逆元
long long modinv(long long a, long long m) {
  long long b = m, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}

#define rep(i, begin_i, end_i) for (ll i = (ll)begin_i; i < (ll)end_i; i++)
// 試験導入
#define irep(i, end_i, begin_i)                                                \\
  for (ll i = (ll)begin_i - 1; i >= (ll)end_i; i--)

long long INF = 1LL << 60;
int main() {
  ll N, K;
  cin >> N >> K;
  vi A(N);
  rep(i, 0, N) cin >> A[i], A[i]--;
  int nxt = 0;
  int a = 0;
  vi visit;
  vi cnt(N, -1);
  while (cnt[nxt] == -1) {
    visit.push_back(nxt + 1);
    cnt[nxt] = a++;
    nxt = A[nxt];
  }
  if (K < visit.size()) {
    cout << visit[K] << endl;
    return 0;
  } else {
    int loopsize = visit.size() - cnt[nxt];
    int l = visit.size() - loopsize;
    K -= l;
    K %= loopsize;
    cout << visit[K + l] << endl;
  }
  return 0;
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
using vll = vector<ll>;
using vvll = vector<vll>;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vb = vector<bool>;
using pii = pair<int, int>;
using vpii = vector<pair<int, int>>;

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

// pow(llpow,modpow)
template <class T> ll llpow(ll x, T n) {
  ll ans = 1;
  if (x == 0)
    ans = 0;
  while (n) {
    if (n & 1)
      ans *= x;
    x *= x;
    n >>= 1;
  }
  return ans;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
// 最大公約数
template <class T> inline T gcd(T x, T y) {
  if (y == 0)
    return x;
  else {
    return gcd(y, x % y);
  }
}
// 最小公倍数
template <class T> inline T lcm(T x, T y) { return x / gcd(x, y) * y; }
// 逆元
long long modinv(long long a, long long m) {
  long long b = m, u = 1, v = 0;
  while (b) {
    long long t = a / b;
    a -= t * b;
    swap(a, b);
    u -= t * v;
    swap(u, v);
  }
  u %= m;
  if (u < 0)
    u += m;
  return u;
}

#define rep(i, begin_i, end_i) for (ll i = (ll)begin_i; i < (ll)end_i; i++)
// 試験導入
#define irep(i, end_i, begin_i)                                                \\
  for (ll i = (ll)begin_i - 1; i >= (ll)end_i; i--)

long long INF = 1LL << 60;
int main() {
  ll N, K;
  cin >> N >> K;
  vi A(N);
  rep(i, 0, N) cin >> A[i], A[i]--;
  vi visit(N, -1);
  ll cnt = 0;
  ll p = 0;
  vi idx;
  while (visit[p] == -1) {
    idx.push_back(p);
    // 最初は1回目
    visit[p] = ++cnt;
    p = A[p];
  }
  if (K < idx.size()) {
    cout << idx[--K] + 1 << endl;
    return 0;
  } else {
    int loopsize = visit.size() - visit[p];

    cout << loopsize << endl;
    vi loop;
    for (int itr = idx.size() - loopsize; itr < idx.size(); itr++) {
      loop.push_back(visit[itr]);
    }
    for (int i : loop)
      cout << i;
    cout << endl;

    K -= visit[p] - 1;
    K--;
    K %= loopsize;
    cout << loop[K] << endl;
  }

  return 0;
}