拓撲排序

const int M = 100005;
int a[M];
int indegree[M];
int n,m,u,v;
vector<int> G[M];
void topo()
{
    memset(indegree,0,sizeof(indegree));
    memset(a,0,sizeof(a));
    for(int i = 1; i <= n ; i++){
        for(int j = 0 ; j <= G[i].size(); j++){
            indegree[G[i][j]]++;
        }
    }
    queue<int> q;
    while(!q.empty())
        q.pop();
    int cnt = 1;
    for(int i = 1;i <= n ; i++){
        if(!indegree[i])
            q.push(i);
    }
    while(!q.empty()){
        int u = q.front();
        q.pop();
        a[cnt++] = u;
        for(int i = 0 ; i <G[u].size(); i++){
            int v = G[u][i];
            indegree[v]--;
            if(!indegree[v])
                q.push(v);
        }
    }
    if(cnt == n){
        for(int i = 1; i < cnt; i++)
            printf(" %d", a[i]);
        printf("%d\n",a[cnt]);
    }
    else printf("This is a cycle\n");
}
int main()
{
    while(~scanf("%d%d",&n,&m)){
　　　　　for(int i = 1; i <= n ; i++)
　　　　　　G[i].clear();
        for(int i = 1; i <= m ; i++){
            scanf("%d%d",&u,&v);
            G[u].push_back(v);
        }
        topo();
    }
    return 0;
}