HDU 1325 Is It A Tree?

//数据：
0 0
1 1 0 0
1 2 2 1 0 0
1 2 2 3 3 4 4 1 0 0
1 2 2 3 3 1 5 6 0 0
1 2 2 3 4 3 0 0
1 2 2 3 5 6 0 0
2 3 0 0

//答案：
Case 1 is a tree.
Case 2 is a tree.
Case 3 is not a tree.//环
Case 4 is not a tree.//环
Case 5 is not a tree.
Case 6 is not a tree.//节点3入度大于1
Case 7 is not a tree.//两棵树
Case 8 is a tree.

/*
ID:muller8
Name: 1325 Is It A Tree?
Reference: 并查集
*/

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <vector>
#include <cmath>
#include <vector>
using namespace std;
#define MAXN 100010
#define INF 1e9

int fa[MAXN];
int in[MAXN];
bool visit[MAXN];
vector<int> vec;    //保存所有的节点

void init(){
memset(in,0,sizeof(in));
memset(visit,false,sizeof(visit));
vec.clear();
for(int i=0;i<MAXN; ++i)
fa[i] = i;
}

int find(int n){
if(fa
!=n)
fa
= find(fa
);
return fa
;
}

int main(){
int x,y;
bool flag = true;
int Case = 0;
init();
while(~scanf("%d%d", &x,&y)){
if(x<0&&y<0)    break;
if(0==x&&0==y){
if(flag){
if(vec.size()){
int tmp = find(vec[0]);
for(int i=1; i<vec.size(); ++i) //判断是否只有一棵树
if(tmp!=find(vec[i])){
flag = false;
break;
}
}
}
printf("Case %d is %s\n", ++Case, flag?"a tree.":"not a tree.");
flag = true;
init();
continue;
}

//用一个vector来保存所有出现过的节点
if(!visit[x]){
visit[x] = true;
vec.push_back(x);
}
if(!visit[y]){
visit[y] = true;
vec.push_back(y);
}

if(!flag)   continue;   //如果已经不是一棵树了，也就不必要太多的操作
if(x==y)    continue;   //如果是同一个点也不用进行运算，当然这里是防止坑爹数据的
int fx, fy;
fx = find(x);
fy = find(y);
if(fx == fy){
flag = false;
continue;
}
else{
fa[fx] = fy;
in[y]++;
}
if(in[y]>1) flag = false;       //入度大于1，flag为否
}
return 0;
}