HDU-3342-Legal or Not

#include <iostream>
#include <memory.h>
#include <vector>
#include <map>
#include <algorithm>
#include <cstdio>
#include <math.h>
#include <queue>

using namespace std;

typedef long long LL;

const int MAXN = 500 + 10;

vector<int> Group[MAXN];
int in[MAXN];
int n, m;

bool topo()
{
    vector<int> res;
    priority_queue<int> que;
    for (int i = 0; i <= n - 1;i++)
        if (in[i] == 0)
            que.push(-i);

    while (!que.empty())
    {
        int now = -que.top();
        que.pop();
        res.push_back(now);
        for (auto x : Group[now])
            if (--in[x] == 0)
                que.push(-x);
    }

    if (res.size() != n)
        return false;
    else
        return true;

}

int main()
{
    int l, r;
    while (scanf("%d%d", &n, &m) && n)
    {
        for (int i = 0;i <= n - 1;i++)
            Group[i].clear();
        memset(in, 0, sizeof(in));

        for (int i = 1; i <= m; i++)
        {
            scanf("%d%d", &l, &r);
            Group[l].push_back(r);
            in[r]++;
        }

        if (topo())
            cout << "YES" << endl;
        else
            cout << "NO" << endl;

    }


    return 0;
}