题解

#include <iostream>
#include <cstdio>
#include <vector>
using namespace std;
vector <int> v[200005];
bool vis[200005];
int n, m, a, b, x, y;
bool dfs(int x){
    if(x == b) return 1;
	vis[x] = 1;
	for(int i = 0; i < v[x].size(); i++){
		if(!vis[v[x][i]]){
			if(dfs(v[x][i])) return 1;
		}
	}return 0;
}
int main(){
	scanf("%d%d%d%d", &n, &m, &a, &b);
	for(int i = 1; i <= m; i++){
		scanf("%d%d", &x, &y);
        v[x].push_back(y);
	}
    cout << (dfs(a) ? "Yes" : "No");

	return 0;
}