思路：通过kosaraju算法缩点，缩点后的图无环，所以出度为0的点就是所有点都可以到达的点（出度为0的点只有一个的情况），如果出度为0的点的数量大于1，则输出0.

#include<iostream>
#include<vector>
using namespace std;
struct Edge {
    int to, nextEdge;
};
struct Node {
    int cowSum;
    int out;
};
vector<Node>node_s;
vector<Edge>edge;
vector<Edge>edgeBeside;
vector<int>head;
vector<int>headBeside;
vector<bool>visited;
vector<int>mappingNode;
int N, M;
void AddEdge(int from, int to,
    vector<Edge>&eg,
    vector<int>&hd) {
    Edge add = {to, hd[from]};
    hd[from] = eg.size();
    eg.push_back(add);
}
void DFS(int node, vector<int>&rememberNode,
    const vector<Edge>&eg,
    const vector<int>&hd) {
    visited[node]=1;
    rememberNode.push_back(node);
    for(int i=hd[node];i;i=eg[i].nextEdge){
        if(!visited[eg[i].to]){
            DFS(eg[i].to,rememberNode,eg,hd);
        }
    }
}
int main() {
    ios::sync_with_stdio(0), cin.tie(0);
    cin >> N >> M;
    mappingNode.assign(N+1,0);
    edge.push_back({0, 0});
    edgeBeside.push_back({0, 0});
    head.assign(N + 1, 0);
    headBeside.assign(N + 1, 0);
    visited.assign(N + 1, 0);
    for (int i = 1; i <= M; i++) {
        int from, to;
        cin >> from >> to;
        AddEdge(from, to, edge, head);
        AddEdge(to, from, edgeBeside, headBeside);
    }
    vector<int>rememberNode;
    rememberNode.clear();
    for(int i=1;i<=N;i++){
        vector<int>add;
        add.clear();
        if(!visited[i]){
            DFS(i,add,edge,head);
        }
        for(int j=add.size()-1;j>=0;j--){
            rememberNode.push_back(add[j]);
        }
    }
    visited.assign(N+1,0);
    for(int i=N-1;i>=0;i--){
        vector<int>node_node;
        node_node.clear();
        if(!visited[rememberNode[i]]){
            DFS(rememberNode[i],node_node,edgeBeside,headBeside);
        }else{
            continue;
        }
        for(const int &node:node_node){
            mappingNode[node]=node_s.size();
        }
        node_s.push_back({int(node_node.size()),0});
    }
    for(int i=1;i<=N;i++){
        for(int j=head[i];j;j=edge[j].nextEdge){
            if(mappingNode[i]!=mappingNode[edge[j].to]){
                node_s[mappingNode[i]].out++;
            }
        }
    }
    int Ans=0;
    for(int i=0,length=node_s.size();i<length;i++){
        if(node_s[i].out==0&&Ans!=0){
            cout<<"0";
            return 0;
        }//*/
        if(node_s[i].out==0&&Ans==0){
            Ans=node_s[i].cowSum;
        }
    }
    cout<<Ans;
    return 0;
}