POJ1308 Is It A Tree?

1. 并查集，有点忘了，看维基百科回顾起来了，三个操作，路径压缩、合并、查找父节点。代码很简单，具体参考http://zh.wikipedia.org/wiki/%E5%B9%B6%E6%9F%A5%E9%9B%86

2. WA了好多次，最后发现是当空树的时候也是一棵树，fuck。。。

#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>
#include <cstring>
#include <stack>
#include <queue>
#define maxn 1000
using namespace std;
int p[maxn], flag[maxn];
int getfather(int x)
{
    return p[x] == x ? x : p[x] = getfather(p[x]);
}
int main()
{
    int t = 1, a, b;
    while (cin >> a >> b && a != -1)
    {
        if (!a)
        {
            cout << "Case " << t++ << " is a tree." << endl;
            continue;
        }
        int is = 1, tail = 0;
        for (int i = 1; i < maxn; ++i)
        {
            p[i] = i;
            flag[i] = 0;
        }
        while (a)
        {
            if (is)
            {
                int ra = getfather(a), rb = getfather(b);
                if (ra == rb)
                {
                    is = 0;
                }
                else
                {
                    p[rb] = ra;
                    flag[a] = flag[b] = 1;
                    tail = max(tail, max(a, b));
                }
            }
            cin >> a >> b;
        }
        if (!is)
        {
            cout << "Case " << t++ << " is not a tree." << endl;
        }
        else
        {
            int roots = 0;
            for (int i = 1; i <= tail; i++)
            {
                if (flag[i] && p[i] == i)
                {
                    roots++;
                    if (roots > 1)
                        break;
                }
            }
            if (roots == 1)
                cout << "Case " << t++ << " is a tree." << endl;
            else
                cout << "Case " << t++ << " is not a tree." << endl;
        }

    }
    return 0;
}