POJ 1308 Is It A Tree? 并查集

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

Case 1 is a tree.
Case 2 is a tree.
Case 3 is not a tree.
//思路  没有孤立的节点 没有环就是树
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn=99999;
int a[maxn];//并查集
int rank[maxn];//树高

void init()
{
    for(int i=0;i<maxn;i++)a[i]=i,rank[i]=0;
}

int find(int x)
{
    if(x==a[x])return x;
    return a[x]=find(a[x]);
}

void unit(int x,int y)
{
    x=find(x);
    y=find(y);
    if(x==y)return ;
    if(rank[x]<rank[y])
        a[x]=y;
    else
    {
        a[y]=x;
        if(rank[x]==rank[y])rank[x]++;
    }
}

bool same(int x,int y)
{
    return find(x)==find(y);
}
bool vis[maxn];
int main()
{
    int n,m,i,j,k,t;
    int x,y;int cas=1;
    while(true)
    {
        init();
        int ok=0;//判断是否存在环
        memset(vis,false,sizeof(vis));
        while(cin>>x>>y)
        {
            if(x==-1&&y==-1)return 0;
            if(!x&&!y)break;
            vis[x]=vis[y]=true;
           if(!same(x,y))
                unit(x,y);
           else ok=1;
        }
        int sum=0;
        for(i=1;i<maxn;i++)if(vis[i])
        {
            if(i==a[i])sum++;//判断是否存在孤立的节点
        }
        if(ok||sum>1)printf("Case %d is not a tree.\n",cas++);
        else printf("Case %d is a tree.\n",cas++);

    }

    return 0;
}