POJ-1308-Is It A Tree?

这个题要求你判断所给的点集是否是一棵树，不算难吧，用并查集可以做，注意细节即可，0 0空树也要算树，

代码：

#include<cstdio>#include<cstring>#include<iostream>using namespace std;const int maxn=10011;int p[maxn];bool vis[maxn];int find(int x){    if(p[x]==-1)return x;    return p[x]=find(p[x]);}void Union(int x,int y){    p[min(x,y)]=max(x,y);}int main(){    int a,b,cas=1;    while(scanf("%d%d",&a,&b)!=EOF)    {if(a==-1&&b==-1)    break;if(a==0&&b==0){    printf("Case %d is a tree.\n",cas++);    continue;}memset(p,-1,sizeof(p));memset(vis,0,sizeof(vis));int n=0,ans=0;n=max(a,max(n,b));int x=find(a);int y=find(b);vis[a]=vis[b]=1;if(x==y)    ans++;else    Union(x,y);while(scanf("%d%d",&a,&b)&&(a+b)){    n=max(a,max(n,b));    vis[a]=vis[b]=1;    x=find(a);    y=find(b);    if(x==y)ans++;    elseUnion(x,y);}for(int i=1;i<=n;i++)    if(vis[i]&&p[i]==-1)ans++;if(ans==1)    printf("Case %d is a tree.\n",cas++);else    printf("Case %d is not a tree.\n",cas++);    }    return 0;}