UESTC 916 方老师的分身III --拓扑排序

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <queue>
using namespace std;
#define N 10007

int first[N],in[N],candy[N];
int tot;
struct node
{
    int u,v,next,w;
}G[20006];

void addedge(int u,int v)
{
    G[tot].u = u;
    G[tot].v = v;
    G[tot].w = 1;
    G[tot].next = first[u];
    first[u] = tot++;
}

int main()
{
    int n,m,cnt,i,j,x;
    int a,b;
    queue<int> que;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        memset(in,0,sizeof(in));
        while(!que.empty())
            que.pop();
        memset(first,-1,sizeof(first));
        tot = 0;
        for(i=0;i<m;i++)
        {
            scanf("%d%d",&a,&b);
            addedge(b,a);
            in[a]++;
        }
        for(i=1;i<=n;i++)
        {
            if(in[i] == 0)
            {
                que.push(i);
                candy[i] = 888;
            }
        }
        cnt = 0;
        while(!que.empty())
        {
            x = que.front();
            que.pop();
            cnt++;
            for(int e=first[x];e!=-1;e=G[e].next)
            {
                in[G[e].v]--;
                if(in[G[e].v] == 0)
                {
                    que.push(G[e].v);
                    candy[G[e].v] = candy[x]+1;
                }
            }
        }
        if(cnt < n)
            puts("-1");
        else
        {
            int sum = 0;
            for(i=1;i<=n;i++)
                sum += candy[i];
            printf("%d\n",sum);
        }
    }
    return 0;
}