Uva 10763 - Foreign Exchange

Problem E
Foreign Exchange
Input: standard input
Output: standard output
Time Limit: 1 second

切都是水题，本来可以好好动脑的，为什么天杀的就让我贪容易混着STL将它过了呢？

#include<iostream>
#include<cstring>
#include<map>
#define SIZE 1000002
using namespace std;
multimap<int, int>exchange;
int main()
{
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    #endif
    int n;
    while(cin>>n && n)
    {
        exchange.clear();
        for(int i=0; i<n; ++i)
        {
            int from, to;
            cin>>from>>to;
            bool flag = true;
            for(multimap<int, int>::iterator i = exchange.find(to); i != exchange.end() && i->first == to; ++i)
            {
                if(i->second == from)
                {
                    exchange.erase(i);
                    flag = false;
                    break;
                }
            }
            if(flag == true) exchange.insert(make_pair(from, to));
        }
        if(exchange.empty()) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}