HDU 2094产生冠军(set思维题)

#include<iostream>
#include<set>
using namespace std;
set<string>s1;
set<string>s2;
int main()
{
    int n;
    while(cin>>n)
    {
        if(n==0)
            break;
        s1.clear();
        s2.clear();
        string a,b;
        for(int i=1;i<=n;i++)
        {
            cin>>a>>b;
            s1.insert(a);
            s2.insert(b);
            s1.insert(b);
        }
        if(s1.size()-s2.size()==1)
            cout<<"Yes"<<endl;
        else
            cout<<"No"<<endl;
    }
}