hdu 3342 Legal or Not

#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
#include<algorithm>
using namespace std;
int rudu[105];
vector<int>abc[105];
int main()
{
    int n, m, i, x, y, j, k;
    while (~scanf("%d%d", &n, &m))
    {
        if (n == 0 && m == 0) break;
        int sum = 0;
        memset(rudu, 0, sizeof(rudu));
        for (i = 0; i < 104; i++) abc[i].clear();
        for (i = 0; i < m; i++)
        {
            scanf("%d%d", &x, &y);
            rudu[y]++;
            abc[x].push_back(y);
        }
        for (i = 0; i < n; i++)
        {
            for (j = 0; j < n; j++)
            {
                if (rudu[j] == 0)
                {
                    sum++;
                    rudu[j]--;
                    for (k = 0; k < abc[j].size(); k++) rudu[abc[j][k]]--;
                    break;
                }
            }
        }
        if (sum == n) printf("YES\n");
        else printf("NO\n");
    }
    return 0;
}