#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define ss(x) scanf("%d",&x);
const int maxn=500000+10;
int vis[maxn];
int main()
{
while(true)
{
memset(vis,0,sizeof(vis));
int n;
ss(n);if(n==0) break;
int a,b,low=maxn,up=0;
rep(i,1,n) {ss(a);ss(b);low=min(low,min(a,b));up=max(up,max(a,b));vis[a]--;vis[b]++;}
bool ok=true;
rep(i,low,up) {if(vis[i]!=0) {ok=false;break;}}
if(ok) puts("YES");
else puts("NO");
}
return 0;
}
UVA 10763 Foreign Exchange(快速检索)
最新推荐文章于 2021-06-24 18:07:17 发布