Problem p02684 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define ld long double
#define forn(low, high, i) for (i = low; i < high; i++)
#define forrev(high, low, i) for (i = high; i >= low; i--)
#define all(v) v.begin(), v.end()
#define decimal_digits cout << fixed << setprecision(15);

string to_string(string s) { return '"' + s + '"'; }
string to_string(char s) { return string(1, s); }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A> string to_string(A);
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A> string to_string(A v) {
  bool f = 1;
  string r = "{";
  for (const auto &x : v) {
    if (!f)
      r += ", ";
    f = 0;
    r += to_string(x);
  }
  return r + "}";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define pr(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

#define GOOGLE 0
#define MULTIPLE_TC 0

const int inf = 1e9;
const long long INF = 1e18;
const int mod = 1e9 + 7;
const int bit32 = log2(inf) + 3;
const int bit64 = log2(INF) + 3;

inline int add(long long a, long long b) {
  a += b;
  if (a >= mod)
    a -= mod;
  return a;
}
inline int sub(long long a, long long b) {
  a -= b;
  if (a < 0)
    a += mod;
  return a;
}
inline int mul(long long a, long long b) {
  a %= mod;
  b %= mod;
  return (int)((long long)a * b % mod);
}
inline int modexpo(long long a, long long b) {
  int res = 1;
  while (b > 0) {
    if (b & 1)
      res = mul(res, a);
    a = mul(a, a);
    b /= 2;
  }
  return res;
}
inline int divide(long long a, long long b) {
  return mul(a, modexpo(b, mod - 2));
}
inline int inv(long long a) { return modexpo(a, mod - 2); }

clock_t time_p = clock();

void ktj() {
  time_p = clock() - time_p;
  cerr << "Time elapsed : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}

void pre() { // Reset each global variable (esp. for graphs)
}

const int N = 2e5 + 10;
int a[N];

void solve() {
  long long n, k;
  int i, j;
  cin >> n >> k;
  for (i = 1; i <= n; i++) {
    cin >> a[i];
  }
  int u = 1, step = 1, sub = 0, p = 0, cyc = 0;
  map<int, int> m, par;
  par[u] = p;
  vector<int> res;
  while (1) {
    if (m.find(u) != m.end()) {
      while (p != u) {
        res.push_back(p);
        p = par[p];
      }
      res.push_back(u);
      sub = m[u] - 1;
      break;
    }
    m[u] = step;
    par[u] = p;
    p = u;
    u = a[u];
    step += 1;
  }
  reverse(all(res));
  cyc = res.size();
  k -= sub;
  k %= cyc;
  cout << res[k] << '\n';
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  pre();

  int t = 1, T;

  if (MULTIPLE_TC)
    cin >> t;

  for (T = 1; T <= t; T++) {
    if (GOOGLE)
      cout << "Case #" << T << ": ";
    solve();
  }

  ktj();
}


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

#define ll long long
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define ld long double
#define forn(low, high, i) for (i = low; i < high; i++)
#define forrev(high, low, i) for (i = high; i >= low; i--)
#define all(v) v.begin(), v.end()
#define decimal_digits cout << fixed << setprecision(15);

string to_string(string s) { return '"' + s + '"'; }
string to_string(char s) { return string(1, s); }
string to_string(const char *s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A> string to_string(A);
template <typename A, typename B> string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A> string to_string(A v) {
  bool f = 1;
  string r = "{";
  for (const auto &x : v) {
    if (!f)
      r += ", ";
    f = 0;
    r += to_string(x);
  }
  return r + "}";
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#define pr(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());

#define GOOGLE 0
#define MULTIPLE_TC 0

const int inf = 1e9;
const long long INF = 1e18;
const int mod = 1e9 + 7;
const int bit32 = log2(inf) + 3;
const int bit64 = log2(INF) + 3;

inline int add(long long a, long long b) {
  a += b;
  if (a >= mod)
    a -= mod;
  return a;
}
inline int sub(long long a, long long b) {
  a -= b;
  if (a < 0)
    a += mod;
  return a;
}
inline int mul(long long a, long long b) {
  a %= mod;
  b %= mod;
  return (int)((long long)a * b % mod);
}
inline int modexpo(long long a, long long b) {
  int res = 1;
  while (b > 0) {
    if (b & 1)
      res = mul(res, a);
    a = mul(a, a);
    b /= 2;
  }
  return res;
}
inline int divide(long long a, long long b) {
  return mul(a, modexpo(b, mod - 2));
}
inline int inv(long long a) { return modexpo(a, mod - 2); }

clock_t time_p = clock();

void ktj() {
  time_p = clock() - time_p;
  cerr << "Time elapsed : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}

void pre() { // Reset each global variable (esp. for graphs)
}

const int N = 2e5 + 10;
int a[N];

void solve() {
  long long n, k;
  int i, j;
  cin >> n >> k;
  for (i = 1; i <= n; i++) {
    cin >> a[i];
  }
  int u = 1, step = 1, sub = 0, p = 0, cyc = 0;
  map<int, int> m, par;
  par[u] = p;
  vector<int> res;
  while (1) {
    if (m.find(u) != m.end()) {
      while (p != u) {
        res.push_back(p);
        p = par[p];
      }
      res.push_back(u);
      sub = m[u] - 1;
      break;
    }
    m[u] = step;
    par[u] = p;
    p = u;
    u = a[u];
    step += 1;
  }
  reverse(all(res));
  cyc = res.size();
  if (k > sub) {
    k -= sub;
    k %= cyc;
    cout << res[k] << '\n';
  } else {
    int u = 1;
    for (i = 1; i <= k; i++) {
      u = a[u];
    }
    cout << u << '\n';
  }
}

int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);

  pre();

  int t = 1, T;

  if (MULTIPLE_TC)
    cin >> t;

  for (T = 1; T <= t; T++) {
    if (GOOGLE)
      cout << "Case #" << T << ": ";
    solve();
  }

  ktj();
}