Is It A Tree? poj 1308

#include<stdio.h>
#include<string.h>
#include<iostream>
#include<algorithm>
#include<math.h>
#define maxn 110
#define INF 0xfffffff
using namespace std;

int father[maxn], v[maxn], flag;

int Find(int x)
{
    while(x!=father[x])
        x = father[x];

    return x;
}

void solve(int a, int b)
{
    v[a] = v[b] = 1;

    if(a == b)  flag = 1;
    else
    {
        int ra = Find(a);
        int rb = Find(b);

        if(ra == rb) flag = 1;
        else father[rb] = ra;
    }
}

int main()
{
    int a, b, fist, t=1;

    while(scanf("%d %d", &a, &b), a!=-1 || b!=-1)
    {
        fist = a;

        memset(v, 0, sizeof(v));

        for(int i=1; i<maxn; i++)
            father[i] = i;

        flag = 0;

        if(a==0 && b==0)
        {
            printf("Case %d is a tree.\n",t++);//空树也是树；
            continue;
        }

        solve(a, b);

        while(scanf("%d %d", &a, &b), a+b)
        {
            solve(a, b);
        }

        for(int i=1; i<maxn; i++)
        {
            if(Find(i)!=Find(fist) && v[i])
            {
                flag = 1;
                break;
            }
        }

        if(!flag)  printf("Case %d is a tree.\n", t++);

        else printf("Case %d is not a tree.\n", t++);

    }
    return 0;
}