Language: C++

#include <bits/stdc++.h>
#define REP(i, n)     for(int i = 0;i < n;i++)
#define REPR(i, n)    for(int i = n;i >= 0;i--)
#define FOR(i, m, n)  for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define VRSORT(v) sort(v.begin(), v.end(),greater<int>());
#define ALL(X) (X).begin(),(X).end()
#define INF 99999999
#define M_PI 3.14159265358979323846
#define MOD 1000000007

#ifdef __LOCAL
    #define DBG(X) cout << #X << " = " << (X) << endl;
    #define SAY(X) cout << (X) << endl;
#else
    #define DBG(X)
    #define SAY(X)
#endif

#ifdef __LOCAL
    #include <filesystem>
    namespace fs = std::filesystem;
#endif

using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

int dx[]={1, 0, -1, 0, 0};
int dy[]={0, 1, 0, -1, 0};
const int INT_INF = (int)(2e9);
const ll  LL_INF = (ll)(2e18);
const int NIL = -1;
static mt19937 _g(time(nullptr));
 
std::string pad(int num){ char buffer[4]; std::snprintf(buffer, sizeof(buffer), "%03d", num); return buffer;}
inline ll randint(ll a, ll b) { ll w = (_g() << 31LL) ^ _g(); return a + w % (b - a + 1); }
inline void fast_io() { ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr); };
template<typename T> inline T sign(T x) { return T(x > 0) - T(x < 0); }
template<typename T, typename S> inline ostream& operator<<(ostream& os, const pair<T, S> p) { cout << "[" << p.first << ";" << p.second << "]"; return os; }
template<typename T> inline ostream& operator<<(ostream& os, const vector<T>& v) { for (auto el : v) os << el << " "; return os; }
template<typename T> inline T fetch() { T ret; cin >> ret; return ret; }
template<typename T> inline vector<T> fetch_vec(int sz) { vector<T> ret(sz); for (auto& elem : ret) cin >> elem; return ret; }


int N,M;
int S,T;
// vector<int> A,B;
map<int,vector<int> > toList;
// map<int,vector<int> > to2List;
// map<int,vector<int> > to3List;

void input(){
  #ifdef __LOCAL
      fs::path p = __FILE__;
      fs::path input,output;
      input = output = p.parent_path();
      input += string("/input/") + string(p.stem()) + string(".txt");
      output += string("/output/") + string(p.stem()) + string(".txt");
      freopen(input.c_str(), "r", stdin);
      freopen(output.c_str(), "w", stdout);
  #endif

  fast_io();
  cin >> N >> M;
  for (int i = 0;i < M;i++)
  {
    int a,b;
    cin >> a >> b;
    a--;b--;
    toList[a].push_back(b);
  }
  cin >> S >> T;
  S--;
  T--;
  // for (int i = 0; i < N; i++)
  // {
  //   for(auto x:toList[i]) for(auto y:toList[x]) to2List[i].push_back(y);
  // }

  // for (int i = 0; i < N; i++)
  // {
  //   for(auto x:to2List[i]) for(auto y:toList[x]) to3List[i].push_back(y);
  // }
}

int solve(){
  vector<bool> visited(N,false);
  queue<pii> q;
  q.push({S,0});
  visited[S=true;
  int now,dist;
  while(!q.empty()){
    tie(now,dist) = q.front();
    q.pop();
    // if(dist%3==0 && now==T) {cout << dist/3 << endl; return 0;}
    // if(dist%3==0 && visited[now]) continue;
    // if(dist%3==0) visited[now]=true;
    for(auto next:toList[now]){
      int next_dist = dist+1;
      if(next_dist%3==0 && visited[next]) continue;
      q.push({next,next_dist});
      if(next_dist%3==0 && next==T) {cout << next_dist/3 << endl; return 0;}
      if(next_dist%3==0) visited[next]=true;
    }

  }
  cout << -1 << endl;
  return 0;
}

int main()
{
  input();
  solve();
  return 0;
}


Compiler output:
a.cc: In function 'int solve()':
a.cc:97:17: error: expected ']' before ';' token
   97 |   visited[S=true;
      |                 ^
      |                 ]
a.cc:97:17: warning: ignoring return value of 'std::vector<bool, _Alloc>::reference std::vector<bool, _Alloc>::operator[](size_type) [with _Alloc = std::allocator<bool>; reference = std::vector<bool>::reference; size_type = long unsigned int]', declared with attribute 'nodiscard' [-Wunused-result]
In file included from /usr/include/c++/14/vector:67,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_bvector.h:1086:7: note: declared here
 1086 |       operator[](size_type __n)
      |       ^~~~~~~~