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

// https://ei1333.github.io/luzhiled/
// http://beet-aizu.hatenablog.com/entry/2017/01/04/113233
// http://www.prefield.com/algorithm/
#include <bits/stdc++.h>
constexpr char bn = '\\n';
using namespace std;
using ll = long long;
using ld = long double;
using vl = vector<ll>;
using vd = vector<ld>;
using vs = vector<string>;
using vb = vector<bool>;
using vvl = vector<vector<ll>>;
using vvd = vector<vector<ld>>;
using vvs = vector<vector<string>>;
using vvb = vector<vector<bool>>;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
template <class T> using vc = vector<T>;
template <class T> using vvc = vector<vector<T>>;
using Graph = vector<vector<ll>>;
const ll INF = 1LL << 60;
void init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
const ll tl = 1000000007;
#define REP(i, n) for (ll i = 0; i < n; i++)
#define REREP(i, n) for (ll i = n; i >= 0; i--)
#define FOR(i, a, n) for (ll i = a; i < n; i++)
#define REFOR(i, n, a) for (ll i = n; i >= a; i--)
#define CLR(mat, f) memset(mat, f, sizeof(mat))
#define IN(a, b, x) (a <= x && x <= b)
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end()) // 被り削除
#define debug cout << "line : " << __LINE__ << " debug" << endl;
#define ini(...)                                                               \\
  int __VA_ARGS__;                                                             \\
  in(__VA_ARGS__)
#define inl(...)                                                               \\
  long long __VA_ARGS__;                                                       \\
  in(__VA_ARGS__)
#define ind(...)                                                               \\
  long double __VA_ARGS__;                                                     \\
  in(__VA_ARGS__)
#define ins(...)                                                               \\
  string __VA_ARGS__;                                                          \\
  in(__VA_ARGS__)
#define inc(...)                                                               \\
  char __VA_ARGS__;                                                            \\
  in(__VA_ARGS__)
void in() {}
template <typename T, class... U> void in(T &t, U &...u) {
  cin >> t;
  in(u...);
}
void out() { cout << endl; }
template <typename T, class... U> void out(const T &t, const U &...u) {
  cout << t;
  if (sizeof...(u))
    cout << " ";
  out(u...);
}
template <typename T> void die(T x) {
  out(x);
  exit(0);
}
#define in1(A) REP(i, A.size()) in(A[i]);
#define in2(A, B) REP(i, A.size()) in(A[i], B[i]);
#define in3(s, t, u)                                                           \\
  REP(i, sz(s)) { in(s[i], t[i], u[i]); }
#define in4(s, t, u, v)                                                        \\
  REP(i, sz(s)) { in(s[i], t[i], u[i], v[i]); }
#define each(x, v) for (auto &x : v)
#define all(v) (v).begin(), (v).end()
#define sz(v) ((int)(v).size())
struct Point {
  ld x, y;
};
ld dist(Point a, Point b) {
  return sqrt(abs(a.x - b.x) * abs(a.x - b.x) +
              abs(a.y - b.y) * abs(a.y - b.y));
} // ２点間の距離
ll gcd(ll a, ll b) { return b != 0 ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll modpow(ll a, ll n, ll mod) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
ll fact(ll n) {
  if (n < 2)
    return 1;
  return (n * fact(n - 1)) % tl;
} // 階乗
inline ll updiv(ll a, ll b) { return (a + b - 1) / b; } // 切り上げ
template <typename T, typename U> ll ceil(T a, U b) { return (a + b - 1) / b; }
template <class A, class B> inline bool chmax(A &a, const B &b) {
  return b > a && (a = b, true);
}
template <class A, class B> inline bool chmin(A &a, const B &b) {
  return b < a && (a = b, true);
}
#define YES(n) ((n) ? "YES" : "NO")
#define Yes(n) ((n) ? "Yes" : "No")
#define yes(n) ((n) ? "yes" : "no")
//-------------------------------------------------------------------------------------------------
int main() {
  init();
  ll ans = 0;
  inl(N, k);
  k++;
  vl A(N);
  in1(A);
  map<ll, ll> T;
  vl roop;
  roop.pb(1);
  T[1] = 1;
  ll now = A[0];
  while (!T[now]) {
    T[now] = 1;
    roop.pb(now);
    now = A[now - 1];
  }
  ll p;
  ll n = roop.size();
  REP(i, n) if (now == roop[i]) p = i;
  if (k <= p) {
    out(roop[k - 1]);
    return 0;
  }
  ll r = n - p;
  k -= p;
  k = k % r - 1;
  if (k == -1)
    k = r - 1;
  out(roop[p + k]);
}


Error: WA

Fixed submission:

// https://ei1333.github.io/luzhiled/
// http://beet-aizu.hatenablog.com/entry/2017/01/04/113233
// http://www.prefield.com/algorithm/
#include <bits/stdc++.h>
constexpr char bn = '\\n';
using namespace std;
using ll = long long;
using ld = long double;
using vl = vector<ll>;
using vd = vector<ld>;
using vs = vector<string>;
using vb = vector<bool>;
using vvl = vector<vector<ll>>;
using vvd = vector<vector<ld>>;
using vvs = vector<vector<string>>;
using vvb = vector<vector<bool>>;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
template <class T> using vc = vector<T>;
template <class T> using vvc = vector<vector<T>>;
using Graph = vector<vector<ll>>;
const ll INF = 1LL << 60;
void init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
const ll tl = 1000000007;
#define REP(i, n) for (ll i = 0; i < n; i++)
#define REREP(i, n) for (ll i = n; i >= 0; i--)
#define FOR(i, a, n) for (ll i = a; i < n; i++)
#define REFOR(i, n, a) for (ll i = n; i >= a; i--)
#define CLR(mat, f) memset(mat, f, sizeof(mat))
#define IN(a, b, x) (a <= x && x <= b)
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end()) // 被り削除
#define debug cout << "line : " << __LINE__ << " debug" << endl;
#define ini(...)                                                               \\
  int __VA_ARGS__;                                                             \\
  in(__VA_ARGS__)
#define inl(...)                                                               \\
  long long __VA_ARGS__;                                                       \\
  in(__VA_ARGS__)
#define ind(...)                                                               \\
  long double __VA_ARGS__;                                                     \\
  in(__VA_ARGS__)
#define ins(...)                                                               \\
  string __VA_ARGS__;                                                          \\
  in(__VA_ARGS__)
#define inc(...)                                                               \\
  char __VA_ARGS__;                                                            \\
  in(__VA_ARGS__)
void in() {}
template <typename T, class... U> void in(T &t, U &...u) {
  cin >> t;
  in(u...);
}
void out() { cout << endl; }
template <typename T, class... U> void out(const T &t, const U &...u) {
  cout << t;
  if (sizeof...(u))
    cout << " ";
  out(u...);
}
template <typename T> void die(T x) {
  out(x);
  exit(0);
}
#define in1(A) REP(i, A.size()) in(A[i]);
#define in2(A, B) REP(i, A.size()) in(A[i], B[i]);
#define in3(s, t, u)                                                           \\
  REP(i, sz(s)) { in(s[i], t[i], u[i]); }
#define in4(s, t, u, v)                                                        \\
  REP(i, sz(s)) { in(s[i], t[i], u[i], v[i]); }
#define each(x, v) for (auto &x : v)
#define all(v) (v).begin(), (v).end()
#define sz(v) ((int)(v).size())
struct Point {
  ld x, y;
};
ld dist(Point a, Point b) {
  return sqrt(abs(a.x - b.x) * abs(a.x - b.x) +
              abs(a.y - b.y) * abs(a.y - b.y));
} // ２点間の距離
ll gcd(ll a, ll b) { return b != 0 ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }
ll modpow(ll a, ll n, ll mod) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
ll fact(ll n) {
  if (n < 2)
    return 1;
  return (n * fact(n - 1)) % tl;
} // 階乗
inline ll updiv(ll a, ll b) { return (a + b - 1) / b; } // 切り上げ
template <typename T, typename U> ll ceil(T a, U b) { return (a + b - 1) / b; }
template <class A, class B> inline bool chmax(A &a, const B &b) {
  return b > a && (a = b, true);
}
template <class A, class B> inline bool chmin(A &a, const B &b) {
  return b < a && (a = b, true);
}
#define YES(n) ((n) ? "YES" : "NO")
#define Yes(n) ((n) ? "Yes" : "No")
#define yes(n) ((n) ? "yes" : "no")
//-------------------------------------------------------------------------------------------------
int main() {
  init();
  ll ans = 0;
  inl(n, k);
  vl A(n);
  in1(A);
  map<ll, ll> T;
  ll now = A[0];
  ll cnt = 0;
  T[1] = 0;
  while (!T[now]) {
    cnt++;
    out(now);
    T[now] = cnt;
    now = A[now - 1];
    if (now == 1)
      break;
  }
  out(now);
  // out(cnt);
  ll p;
  T[now] = cnt + 1;
  if (now == 1) {

    T[now] = cnt + 1;

    p = (k % (cnt + 1));
    if (p == 0) {
      out(1);
    }
    // if(!p) p= (cnt-T[now]+1);
    each(x, T) {
      if (x.second == p) {
        out(x.first);
        return 0;
      }
    }
  } else {
    if (T[now] >= k) {
      each(x, T) {
        if (x.second == k) {
          out(x.first);
          return 0;
        }
      }
    } else {
      p = ((k - T[now] + 1) % (cnt - T[now] + 1));
      if (!p)
        p = (cnt - T[now] + 1);

      p += T[now] - 1;
      // out(p, T[now],cnt-T[now]+1);
      each(x, T) {
        if (x.second == p) {
          out(x.first);
          return 0;
        }
      }
    }
  }

  // out();
}