HDU-1272 小希的迷宫 （并查集、判断图是否为树）

 1 # include<iostream>
 2 # include<cstdio>
 3 # include<algorithm>
 4 # include<cstring>
 5 # include<set>
 6 using namespace std;
 7 const int N=1000005;
 8 struct edge
 9 {
10     int fr,to;
11 };
12 edge e[N];
13 int pre[N],n,cnt;
14 int mark[N];
15 void add(int u,int v)
16 {
17     e[cnt].fr=u;
18     e[cnt].to=v;
19     ++cnt;
20 }
21 int find(int x)
22 {
23     int u=x;
24     while(pre[u]!=-1)
25         u=pre[u];
26     int v;
27     while(pre[x]!=-1){
28         v=x;
29         x=pre[x];
30         pre[v]=u;
31     }
32     return u;
33 }
34 bool ok()
35 {
36     memset(pre,-1,sizeof(pre));
37     for(int i=0;i<cnt;++i){
38         int u=find(e[i].fr);
39         int v=find(e[i].to);
40         //cout<<e[i].fr<<‘ ‘<<e[i].to<<‘ ‘<<u<<‘ ‘<<v<<endl;
41         if(u==v)
42             return false;
43         pre[u]=v;
44     }
45     return true;
46 }
47 int main()
48 {
49     int a,b;
50     while(scanf("%d%d",&a,&b))
51     {
52         if(a==-1&&b==-1)
53             break;
54         if(a==0&&b==0){
55             printf("Yes\n");
56             continue;
57         }
58         cnt=0;
59         memset(mark,0,sizeof(mark));
60         int num=0;
61         mark[a]=mark[b]=1;
62         num+=2;
63         add(a,b);
64         //add(b,a);
65         while(scanf("%d%d",&a,&b),a+b){
66             if(!mark[a])
67                 ++num;
68             if(!mark[b])
69                 ++num;
70             mark[a]=mark[b]=1;
71             add(a,b);
72             //add(b,a);
73         }
74         //cout<<cnt/2<<‘ ‘<<num-1<<endl;
75         if(cnt==num-1&&ok())
76             printf("Yes\n");
77         else
78             printf("No\n");
79     }
80     return 0;
81 }