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 1000using 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;}