poj 1308 Is It A Tree?

#include<stdio.h>
#define N 1000
int f[N+10],t[N+10],c[N+10];
int find(int a)
{
    if(a!=f[a])
        f[a]=find(f[a]);
    return f[a];
}
int mix(int a,int b)
{
    int x,y;
    x=find(a);
    y=find(b);
    if(x==y)
        return 0;
    f[x]=y;
    return 1;
}
int main()
{
    int i,j=0,n,m,a,b,flag;
    while(scanf("%d%d",&m,&n)!=EOF){
        if(m<0&&n<0)                       //当输入两个负数时结束
            break;
        j++;
        if(m==0&&n==0){
            printf("Case %d is a tree.\n",j);
            continue;
        }
        for(i=1;i<=N;i++){
            f[i]=i;
            c[i]=t[i]=0;
        }
        flag=1;
        while(m!=0&&n!=0){
            t[m]=t
=1;
            c
++;           //计算入度
            if(c
>1)
                flag=0;
            a=mix(m,n);      //判断是否会形成环  
            if(!a)
                flag=0;
            scanf("%d%d",&m,&n);
        }
        if(flag){
            b=0;
            for(i=1;i<=N;i++){
                if(t[i]&&f[i]==i)
                    b++;                   //计算根结点数目
                if(b>1)
                    break;
            }
        }
        if(flag&&b==1)
            printf("Case %d is a tree.\n",j);
        else
            printf("Case %d is not a tree.\n",j);
    }
    return 0;
}