BFS ( Adj List )

#include <iostream>
#include <algorithm>
#include <numeric>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <climits>
#include <string>
#include <cstdio>
#include <cmath>
#define task "TESTCODE"
using namespace std;
const int N = 1e5;
int x, y, n, s, m, f, Trace[N+1];
vector<vector<int>> Adj;
queue<int> q;
void read()
{
   cin >> n >> m >> s >> f;
   Adj.resize(n + 1);
   for (int i = 1; i <= m; ++ i)
   {
       cin >> x >> y;
       Adj[x].push_back(y);
   }
   fill (Trace, Trace + n + 1, 0);
}
void BFS(int s)
{
    q.push(s);
    Trace[s] = -1;
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        for (int v : Adj[u])
        {
            if (Trace[v] == 0)
            {
                Trace[v] = u;
                q.push(v);
            }
        }
    }
}
void solve()
{
   BFS(s);
   for (int i = 1; i <= n; ++ i)
   {
       if (Trace[i] != 0)
       {
           cout << i << ' ';
       }
   }
   cout << '\n';
   while (f != s)
   {
       cout << f << "<- ";
       f = Trace[f];
   }
   cout << s;
}
int main()
{
   ios_base::sync_with_stdio(false);
   cin.tie(nullptr);
   //freopen(task".INP", "r", stdin);
   //freopen(task".OUT", "w", stdout);
   read();
   solve();
}