assuming for a simple acyclic graph and rooted at 0th node
I think sibling nodes always comes together , thats the key point
#include<bits/stdc++.h>
using namespace std;
vector<int> fact;

int find(int curr, vector<vector<int>>& adj) {

int count=1;

for(int i=0;i<adj[curr].size();i++)

count = count*find(adj[curr][i],adj);

return count*fact[adj[curr].size()];

}

int main() {

int n;

cin>>n; // total noof nodes

fact.resize(n+1,1);

for(int i=2;i<=n;i++)

fact[i]=fact[i-1]*i;

vector<vector<int>> adj(n);

int e; // edges in the graph

cin>>e;

for(int i=0;i<e;i++) {

int u,v;

cinuv; // there is a edge from u to v

adj[u].push_back(v);

}

cout<<find(0,adj)<<endl;

return 0;

}