题目链接：有向图的拓扑排序

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 100010;

int n, m;
int h[N], e[N], ne[N], idx;

int q[N], d[N];

void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}

bool topsort()
{
    int hh = 0, tt = -1;
    for(int i = 1; i <= n; i++)
    {
        // 如果入度为0，添加到队列中
        if(!d[i]) q[++tt] = i;
    }
    
    while(hh <= tt)
    {
        int t = q[hh++];
        for(int i = h[t]; i != -1; i = ne[i])
        {
            int j = e[i];
            d[j] --;
            if(d[j] == 0) q[++tt] = j;
        }
    }
    return tt == n - 1;
    
}

int main()
{
    cin >> n >> m;
    memset(h, -1, sizeof h);
    for(int i = 0; i < m; i ++)
    {
        int a, b;
        cin >> a >> b;
        
        add(a, b);
        // b的入度+1
        d[b] ++;
    }
    if(topsort())
    {
        for(int i = 0; i < n; i++)
        {
            cout << q[i] << ' ';
        }
    }
    else cout << "-1" << endl;
    return 0;
}