HDU 1325 Is It A Tree?(并查集简单题)

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题目链接

并查集简单题，利用并查集来判断无向图的环

#include<iostream>using namespace std;const int maxn=1005;int fa[maxn],in[maxn],out[maxn],vis[maxn];int Find(int x){    if(fa[x]==x) return x;    return fa[x]=Find(fa[x]);}void Merge(int x,int y){    x=Find(x);    y=Find(y);    if(x!=y) fa[y]=x;}int main(){    int x,y,Case=1;    cin.sync_with_stdio(false);    while(cin>>x>>y&&x+y>=0)    {        int Max=-1;        if(x+y==0)        {            cout<<"Case "<<Case++<<" is a tree."<<endl;            continue;        }        for(int i=0;i<maxn;i++) fa[i]=i,in[i]=0,out[i]=0,vis[i]=0;        Merge(x,y);        in[y]++;        out[x]++;        vis[x]=1;        vis[y]=1;        Max=max(Max,max(x,y));        while(cin>>x>>y,x+y)        {            Merge(x,y);            in[y]++;            out[x]++;            vis[x]=1;            vis[y]=1;            Max=max(Max,max(x,y));        }        int cnt=0;        for(int i=0;i<=Max;i++)            if(vis[i]&&fa[i]==i) cnt++;        if(cnt!=1)        {            cout<<"Case "<<Case++<<" is not a tree."<<endl;            continue;        }        int cnt1=0,cnt2=0;        for(int i=0;i<=Max;i++)            if(vis[i]&&in[i]==0) cnt1++;            else if(vis[i]&&in[i]>1) cnt2++;        if(cnt1!=1||cnt2>0)        {            cout<<"Case "<<Case++<<" is not a tree."<<endl;            continue;        }        cout<<"Case "<<Case++<<" is a tree."<<endl;    }    return 0;}

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