HDU 3342 Legal or Not【拓扑排序】

#include<iostream>  
#include<cstdio>  
#include<cstring>  
#include<algorithm>  
using namespace std;
int indegree[1005],queue[1005],head[1005];
struct Edge
{
	int to;
	int next;
} edge[1005];
int main()
{
	int n,m,i,j,k,iq,a,b;
	while(scanf("%d %d",&n,&m)!=EOF&&n&&m)
	{
		memset(head,-1,sizeof(head));
		memset(indegree,0,sizeof(indegree));
		for(i=1;i<=m;i++)
		{
			scanf("%d %d",&a,&b);
			edge[i].to=b;
			edge[i].next=head[a];
			head[a]=i;
			indegree[b]++;			
		}
		iq=0;
		for(i=0;i<n;i++)
		{
			if(indegree[i]==0)
			{
				queue[iq++]=i;
			}
		}
		
		for(i=0;i<iq;i++)
		{
			for(k=head[queue[i]];k!=-1;k=edge[k].next)
			{
				indegree[edge[k].to]--;
				if(indegree[edge[k].to]==0)
				{
					queue[iq++]=edge[k].to;					
				}
			}
		}
		if(iq==n) printf("YES\n");
		else
		printf("NO\n");	
	}
}