HDU2094容器使用之set

#include<iostream>
#include<set>
#include<algorithm>
#include<string>
using namespace std;
int main()
{//freopen("C:\\Users\\Sky\\Desktop\\1.in","r",stdin);
    int n,i;
    string s1,s2;
    set<string>a,b;
    set<string>::iterator it;
    while(cin>>n,n)
    {
         for(i=1;i<=n;i++)
         {
            cin>>s1>>s2;
            a.insert(s1);
            b.insert(s2);
         }
         for(it=b.begin();it!=b.end();it++)
         {
             if(find(a.begin(),a.end(),*it)!=a.end())
                 a.erase(*find(a.begin(),a.end(),*it));
         }

         if(a.size()==1)
                cout<<"Yes"<<endl;
         else
                cout<<"No"<<endl;
     
  /*
  for(it=a.begin();it!=a.end();it++)
         cout<<*it<<endl;*/
         a.clear();  //清空容器
         b.clear();
    }
    return 0;
}