POJ 1308 Is It A Tree?

Description

A tree is a well-known data structure that is either empty (null, void, nothing) or is a set of one or more nodes connected by directed edges between nodes satisfying the following properties. 

There is exactly one node, called the root, to which no directed edges point. 
Every node except the root has exactly one edge pointing to it. 
There is a unique sequence of directed edges from the root to each node. 
For example, consider the illustrations below, in which nodes are represented by circles and edges are represented by lines with arrowheads. The first two of these are trees, but the last is not. 

In this problem you will be given several descriptions of collections of nodes connected by directed edges. For each of these you are to determine if the collection satisfies the definition of a tree or not.

Input

Output

Sample Input

6 8  5 3  5 2  6 4
5 6  0 0

8 1  7 3  6 2  8 9  7 5
7 4  7 8  7 6  0 0

3 8  6 8  6 4
5 3  5 6  5 2  0 0
-1 -1

Sample Output

Case 1 is a tree.
Case 2 is a tree.
Case 3 is not a tree.

//判断是不是树

#include <iostream>
#include <stdio.h>
#include <string>
#include <cstring>
#include <cmath>
#include <algorithm>
#define N 100009
using namespace std;

int a,b;
int fa[N];
int vis[N];

int findfa(int x)
{
    if(x!=fa[x])
    fa[x]=findfa(fa[x]);

    return fa[x];
}

int main()
{
    int ca=1;

    while(~scanf("%d %d",&a,&b))
    {
        if(a==-1 && b==-1) break;

        if(a==0 && b==0)
        {
            printf("Case %d is a tree.\n",ca++);
            continue;
        }

        int fff=a;//任意记录一个结点

        for(int i=0;i<=N;i++)
        {
            fa[i]=i;vis[i]=0;
        }

        int flag=0;

        while(a||b)
        {
            if(a==0 && b==0)
            break;

            vis[a]=1;vis[b]=1;

            int aa=findfa(a);
            int bb=findfa(b);

            if(aa==bb)//如果重复指向，表明存在环，不是树
            flag=1;
            else
            {
                fa[bb]=aa;
            }
           scanf("%d %d",&a,&b);
        }

        int num=0;

//        for(int i=min;i<=max;i++)
//        cout<<"fa["<<i<<"]="<<fa[i]<<endl;

        if(flag)
        printf("Case %d is not a tree.\n",ca++);
        else
        {
            for(int i=0;i<=105;i++)
            {
                if(vis[i] && findfa(i)!=findfa(fff))//判断是不是只有一个树根
                {
                    flag=1;
                    break;
                }
            }

            if(flag==0)
            printf("Case %d is a tree.\n",ca++);
            else
            printf("Case %d is not a tree.\n",ca++);

        }

    }
    return 0;
}

//WH环境日

POJ 1308 Is It A Tree?

原文：http://blog.csdn.net/wust_zjx/article/details/46366433