Problem p02991 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define PB push_back
#define MP make_pair
#define DEBUG printf("%s\n", "debug")
#define fst first
#define snd second
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define SCD(n) scanf("%d", &n)
#define SCD2(m, n) scanf("%d%d", &m, &n)
#define SCD3(m, n, k) scanf("%d%d%d", &m, &n, &k)
#define SCLLD(n) scanf("%lld", &n)
#define SCLLD2(m, n) scanf("%lld%lld", &m, &n)
#define SCLLD3(m, n, k) scanf("%lld%lld%lld", &m, &n, &k)
using namespace std;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
///////////////////////////////////////////////
const ll MOD = 1e9 + 7;

ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

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;
}
///////////////////////////////////////////////
/*🍈( '-'  🍈*/
int main() {
  int N, M;
  cin >> N >> M;
  static bool visited[300005] = {};
  static ll cost[300005] = {};
  vector<vector<int>> E(300005);

  REP(i, M) {
    int u, v;
    cin >> u >> v;
    u *= 3;
    v *= 3;
    E[u].PB(v + 1);
    E[u + 1].PB(v + 2);
    E[u + 2].PB(v);
  }

  queue<int> Q;
  int S, T;
  cin >> S >> T;
  S *= 3;
  T *= 3;
  Q.push(S);
  visited[S] = true;

  while (!Q.empty()) {
    int now = Q.front();
    visited[now] = true;
    Q.pop();

    for (int i = 0; i < E[now].size(); ++i) {
      int next = E[now][i];
      if (visited[next])
        continue;
      cost[next] = cost[now] + 1;
      Q.push(next);
    }
  }

  if (visited[T])
    cout << cost[T] / 3 << endl;
  else
    cout << -1 << endl;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPr(i, n) for (int i = (n)-1; i >= 0; --i)
#define FORq(i, m, n) for (int i = (m); i <= (n); ++i)
#define FORqr(i, m, n) for (int i = (n); i >= (m); --i)
#define PB push_back
#define MP make_pair
#define DEBUG printf("%s\n", "debug")
#define fst first
#define snd second
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x, 0, sizeof(x))
#define FILL(x, y) memset(x, y, sizeof(x))
#define MM(x) memset(x, -1, sizeof(x))
#define ALL(x) (x).begin(), (x).end()
#define SCD(n) scanf("%d", &n)
#define SCD2(m, n) scanf("%d%d", &m, &n)
#define SCD3(m, n, k) scanf("%d%d%d", &m, &n, &k)
#define SCLLD(n) scanf("%lld", &n)
#define SCLLD2(m, n) scanf("%lld%lld", &m, &n)
#define SCLLD3(m, n, k) scanf("%lld%lld%lld", &m, &n, &k)
using namespace std;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
///////////////////////////////////////////////
const ll MOD = 1e9 + 7;

ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }

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;
}
///////////////////////////////////////////////
/*🍈( '-'  🍈*/
int main() {
  int N, M;
  cin >> N >> M;
  static bool visited[300005] = {};
  static ll cost[300005] = {};
  vector<vector<int>> E(300005);

  REP(i, M) {
    int u, v;
    cin >> u >> v;
    u *= 3;
    v *= 3;
    E[u].PB(v + 1);
    E[u + 1].PB(v + 2);
    E[u + 2].PB(v);
  }

  queue<int> Q;
  int S, T;
  cin >> S >> T;
  S *= 3;
  T *= 3;
  Q.push(S);
  visited[S] = true;

  while (!Q.empty()) {
    int now = Q.front();
    visited[now] = true;
    Q.pop();

    for (int i = 0; i < E[now].size(); ++i) {
      int next = E[now][i];
      if (visited[next])
        continue;
      visited[next] = true;
      cost[next] = cost[now] + 1;
      Q.push(next);
    }
  }

  if (visited[T])
    cout << cost[T] / 3 << endl;
  else
    cout << -1 << endl;

  return 0;
}