拓扑排序-判断环路

#include<iostream>
#include<vector>
#include<queue>
using namespace std;
#define MAX 0x3f3f3f3f
const int N = 1e5 + 5;
vector<int> a[N];
queue<int> q;
int n, m;
int in[N], print[N];
bool topoSort()
{
    int x = 0;
    for (int i = 1; i <= n; i++)
    {
        if (in[i] == 0)
        {
            q.push(i);//把所有入度为0的点入队
            print[x++] = i;
        }
    }
    while (!q.empty())
    {
        int i = q.front();
        for (int j = 0; j < a[i].size(); j++)
        {
            if (in[a[i][j]])
            {
                in[a[i][j]]--;//结点i指向的点入度减1
                if (in[a[i][j]] == 0)
                {
                    q.push(a[i][j]);//如果减为0，可以当作拓扑排序的结点
                    //可以用队列queue，也可以用堆栈stack
                    print[x++] = a[i][j];
                }
            }
        }
        q.pop();
    }
    if (x == n) return true;//判断是否有环路
    else return false;
}

int main()
{
    int num1, num2;
    scanf("%d %d", &n, &m);
    for (int i = 0; i < m; i++)
    {
        scanf("%d %d", &num1, &num2);
        a[num1].push_back(num2);//num1->num2
        in[num2]++;//num2入度+1
    }
    if (topoSort())
    {
        for (int i = 0; i < n; i++)
            printf("%d ", print[i]);
    }
    else printf("-1");

    return 0;
}