hdu 1272 小希的迷宫

考察并查集，判断是否成环（是否有同样的父节点），判断连通（是否只有一个根节点）

#include<iostream>#define maxn 100000+5using namespace std;int a,b;int flag;int father[maxn];int sign[maxn];void ready(){for(int i=1;i<maxn;i++) father[i]=i,sign[i]=0;}int dfs(int x){if(father[x]!=x){father[x]=father[father[x]];}return father[x];}void build(int x,int y){if(x==y) flag=0;else{if(dfs(x)==dfs(y)) flag=0;else{father[dfs(x)]=dfs(y);}}}int main(){while(cin>>a>>b){ready();flag=1;if(a==-1&&b==-1){break;}if(!a&&!b) {cout<<"Yes"<<endl;continue;}build(a,b);sign[a]=sign[b]=1;while(cin>>a>>b&&a&&b){build(a,b);sign[a]=sign[b]=1;}int k=0;for(int i=1;i<maxn;i++){if(sign[i]&&dfs(i)==i) k++;}if(k==1&&flag) cout<<"Yes"<<endl;else cout<<"No"<<endl;}return 0;}