poj 1308 Is It A Tree?

水题，判断是否是一棵树，如果一个结点有多个父亲or图中有环就不是。

#include <stdio.h>
#include <string.h>
int fa[1005],v[1005];
int Find(int n)
{
    if(fa[n] == -1)
        return n;
    fa[n] = Find(fa[n]);
        return fa[n];
}
int main()
{
    int ca=1,a,b,i,j,f;
    bool ok;
    while(scanf("%d%d",&a,&b))
    {
        if(a==-1 && b==-1) break;
        if(!a && !b)
        {
            printf("Case %d is a tree.\n",ca++);
            continue;
        }
        ok = 1; i = 0;
        memset(fa,-1,sizeof fa);
        fa[b] = a; v[i] = b;
        if(a == b)
        {
            ok = 0;
            printf("Case %d is not a tree.\n",ca++);
        }
        while(scanf("%d%d",&a,&v[++i]),a&&v[i])
        if(ok)
        {
            if(fa[v[i]] != -1 || Find(a)==v[i])
            {
                ok = 0;
                printf("Case %d is not a tree.\n",ca++);
            }
            fa[v[i]] = a;
        }
        if(ok)
        {
            f = Find(v[0]);
            for(j = 1; j <= i&&Find(v[j]) == f; j++);
            if(j < i)
                printf("Case %d is not a tree.\n",ca++);
            else printf("Case %d is a tree.\n",ca++);
        }
    }
    return 0;
}