#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <map>
using namespace std;
map<pair<int, int >, int > list;
int main()
{
int n, a, b;
while(scanf("%d",&n) != EOF && n)
{
list.clear();
int cnt = 0;
for( int i = 0; i < n; i++)
{
scanf("%d %d",&a , &b);
if(!list[make_pair(a, b)])
list[make_pair(a, b)] = 1;
else
list[make_pair(a, b)]++;
}
map<pair<int, int >,int > :: iterator it;
bool flag = true;
for( it = list.begin(); it != list.end(); it++)
{
int a = it->first.first;
int b = it->first.second;
if(it->second != list[make_pair(b,a)])
{
flag = false;
break;
}
}
//cout<<cnt<<" "<<ans<<endl;
if(flag)
printf("YES\n");
else
printf("NO\n");
}
return 0;
}
UVA 10763
最新推荐文章于 2019-02-11 16:37:01 发布