HDU1325 Is It A Tree?

 /*不能算是并查集，只是用了路径压缩和树的特点， 和1272的区别在于它是有向图，它的父结点是固定的*/ #include <iostream> using namespace std; int main() { int n,m,k=0,s[100005]={0},j=0,i,big; bool f=0; bool flag[100005]={0}; while(cin>>n>>m) { if(m==-1&&n==-1)return 0; if(m==0&&n==0) { k++; int c=0; for(i=1;i<=big;i++){if(flag[i]){c++;flag[i]=0;s[i]=0;}} if(f) cout<<"Case "<<k<<" is not a tree."<<endl; else if(c==0)cout<<"Case "<<k<<" is a tree."<<endl; else if(c!=j+1)cout<<"Case "<<k<<" is not a tree."<<endl; else cout<<"Case "<<k<<" is a tree."<<endl; j=0; f=0; big=0; } else { j++; flag[n]=flag[m]=1; if((m>n?m:n)>big)big=(m>n?m:n); //如果已经有父结点，但是父结点不是n，那就是不树了 if(s[m]!=0&&s[m]!=n)f=1; else s[m]=n; } } return 0; }