HDU 1325 Is It A Tree?

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.

#include <iostream>
#define maxn 100010
using namespace std;
struct  TREE
{
	int mask;
    int root,rudu;//根、入度
}f[maxn];

void init ()//初始化
{
	int i;
	for(i=0;i<maxn;i++)
	{
		f[i].mask=0;
		f[i].rudu=0;
		f[i].root=i;
	}
}

int find(int x)//查找
{
	if(f[x].root!=x)
		f[x].root=find(f[x].root);
	return f[x].root;
}

void joint(int a,int b)//合并
{
	int fa,fb;
	fa=find(a);
	fb=find(b);

	if(fa!=fb)
		f[fb].root=fa;
}

int main()
{
	int N=1;
	int n,m;
	init();
	bool flag =true;


	while(cin>>n>>m)
	{
		if(n<0&&m<0)
			break;
		if(n==0&&m==0)
		{
			cout<<"Case "<<N<<" ";
			int j;
			int num=0;//树根个数
			for(j=1;j<maxn;j++)
            {
                if(f[j].mask!=0&&find(j)==j)//森林
					num++;

                if(f[j].rudu>1)//入度大于1
                {
                    flag=false;
                    break;
                }
            }

			if(num>1)//根数大于1，是森林
				flag=false;

			if(flag==true)
				cout<<"is a tree."<<endl;
			else
				cout<<"is not a tree."<<endl;

			N++;
			init();//初始化
			flag =true;
			continue;
		}
		if((m!=n&&find(n)==find(m))||m==n)
            flag = false;

		else
		{
			f[n].mask=1;
			f[m].mask=1;
			f[m].rudu++;//记录入度
			joint(n,m);
		}
	}
	return 0;
}