图论 + bfs

#include<bits/stdc++.h>
using namespace std;
const int N=2e5+5;
int n,m,a,b,x,y;
vector<int> v[N];
int vis[N];
bool bfs(){
	queue<int> q;
	vis[a]=1;
	q.push(a);
	while(!q.empty()){
		int t=q.front();
		q.pop();
		if(t==b) return true;
		for(int i=0;i<v[t].size();i++){
			int tt=v[t][i];
			if(!vis[tt]){
				vis[tt]=1;
				q.push(tt);
			}
		}
	}
}
int main(){
	cin>>n>>m>>a>>b;
	for(int i=1;i<=m;i++){
		cin>>x>>y;
		v[x].push_back(y);
	}
	if(bfs()) cout<<"Yes";
	else cout<<"No";
	return 0;
}