UVA - 10763 Foreign Exchange【map】

#include<iostream>
#include<map>
#include<set>
using namespace std;
intmain()
{
    int n;
    while(scanf("%d",&n) && n)
    {
        map<int, int> a, b;
        set<int> ans;
        bool flag = false;
        for (int i = 0; i < n; i++)
        {
            int x, y;
            cin >> x >> y;
            a[x] ++; b[y] ++;
            ans.insert(x); ans.insert(y);
        }

       set<int>::iterator it;
       for (it = ans.begin(); it !=ans.end(); it++)
       {
           if(a[*it] != b[*it])
           {
               flag = true;
               break;
           }
       }
       if(flag) cout << "NO" << endl;
       else cout << "YES" << endl;
    }

}