对一个有向图的节点进行拓扑排序，可以用来判断该有向图是否成环，有环则无拓扑序列，无环则有。     

#include <cstdio>
#include <cstring>
#include<iostream>
#include <queue>
using namespace std;
const int maxn = 1e5 + 7;

int n,m, du[maxn], head[maxn], tot,cnt,ans[maxn];
struct node {
    int v, next;
} edge[maxn];
queue<int>q;
void add(int u, int v) {
    edge[tot].v = v;
    edge[tot].next = head[u];
    head[u] = tot++;
}
void init() {
    tot = 0;
    memset(du, 0, sizeof(du));
    memset(head, -1, sizeof(head));
}
void solve() {
    while(!q.empty()) {
        int u = q.front();
        q.pop();
        ans[cnt++]=u;
        for (int i = head[u] ; i != -1 ; i = edge[i].next) {
            du[edge[i].v]--;
            if (!du[edge[i].v]) q.push(edge[i].v);
        }
    }
}
int main() {

    int x,y;
    scanf("%d",&n);
    init();
    for (int i = 1 ; i <= n ; i++) {


      while(scanf("%d",&x)!=EOF&&x!=0){

      add(i,x);
      du[x]++;}
    }
    for (int i = 1 ; i <= n ; i++)
        if (!du[i]) q.push(i);
    solve();
    if(cnt==m)
    {
        for(int i=0;i<cnt;i++)
            cout<<ans[i]<<" ";
        cout<<endl;
    }
    else
        cout<<"有向图成环"<<endl;
    return 0;
}