HDU2094产生冠军

3
Alice Bob
Smith John
Alice Smith
5
a c
c d
d e
b e
a d
0

#include<iostream>
#include<string>
#include<set>
using namespace std;
set<string> A,B;
string s1,s2;
int n;
int main(){
  while(cin>>n&&n){
    while(n--){
      cin>>s1>>s2;
      A.insert(s1);
      A.insert(s2);
      B.insert(s2);
    }
    if(A.size()-B.size()==1){
      cout<<"Yes"<<endl;
    }else{
      cout<<"No"<<endl;
    }
    A.clear();
    B.clear();
  }
  return 0;
}