拓扑排序
#include<iostream>
using namespace std;
#include<vector>
#include<queue>
#include<algorithm>
const int maxn = 1e6 + 5;
int deg[maxn];
vector<int> son[maxn];
vector<int> tuopu;
int n, m;
void dfs() {
queue<int> q;
for (int i = 1; i <= n; i++) {
if (!deg[i]) {
q.push(i);
}
}
while(!q.empty()){
int u = q.front();
tuopu.push_back(u);
q.pop();
for(int i=0;i<son[u].size();i++){
int v = son[u][i];
deg[v]--;
if(deg[v] == 0){
q.push(v);
}
}
}
}
int main() {
cin >> n >> m;
for (int i = 1; i <= m; i++) {
int u, v;
cin >> u >> v;
son[u].push_back(v);
deg[v]++;
}
dfs();
if(tuopu.size() == n){
for(int i=0;i<tuopu.size();i++){
cout << tuopu[i] <<" ";
}
}
else{
cout << "存在环"<<endl;
}
}``