Is It A Tree

  poj 1308

题目大意：

判断给定的数对是否构成一棵树， 

解决：并查集 

 1 #include <iostream> 
 2 #include <cstring>
 3 using namespace std;
 4 int num[10005];
 5 bool outdegree[10005];
 6 bool mark[10005];//标记该结点是树的一个结点
 7 int find(int x)//并查集的查找
 8 {
 9     if(num[x]<0)return x;
10     return num[x]=find(num[x]);
11 }
12 bool merge(int a,int b)//并查集的合并，并非标准合并程序，根据需要改动
13 {
14     if(outdegree[b])return false;//若为树，每个非根节点的出度一定为1，若出度已经为1，则直接判断不为树
15     outdegree[b]=1;
16     mark[a]=mark[b]=1;
17     int fa=find(a);
18     int fb=find(b);
19     num[b]=a;//将b为根的树接到a上
20     return true;
21 }
22 int main()
23 {
24     int a,b,icase=1;
25     while(cin>>a>>b,a!=-1&&b!=-1)
26     {
27                                  
28       if(a==0&&b==0)
29       {
30           cout<<"Case "<<icase++<<" is a tree."<<endl;
31           continue;  
32       }
33       int ok=true,min=10005,max=0,nroot=0;
34       memset(mark,0,sizeof(mark));//这个地方很重要，必须初始化
35       memset(num,0xff,sizeof(num));
36       memset(outdegree,0,sizeof(outdegree));
37       do
38       {
39           if(!merge(a,b)){ok=false;continue;}
40           if(a<min)min=a;//这四句话是找出最大和最小的边的区间范围
41           if(b<min)min=b;
42           if(a>max)max=a;
43           if(b>max)max=b;
44           
45       }while(cin>>a>>b,a||b);
46   
47      if(!ok)cout<<"Case "<<icase++<<" is not a tree."<<endl;
48      else 
49      {
50           for(int i=min;i<=max&&nroot!=2;i++)
51              if(mark[i] && num[i]<0 )nroot++;//数数结点中根的个数
52           if(nroot==1)cout<<"Case "<<icase++<<" is a tree."<<endl;
53           else cout<<"Case "<<icase++<<" is not a tree."<<endl;
54       }
55     }
56   //  system("pause");
57     return 0;
58 }