範例程式碼 uva10763
//uva10763
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define N 500005
using namespace std;
typedef pair<int, int> PII;
int n;
PII a[N], b[N];
int main(void) {
int i;
while(scanf("%d", &n) == 1 && n) {
for(i = 0; i < n; ++i) {
scanf("%d%d", &a[i].first, &a[i].second);
b[i].first = a[i].second;
b[i].second = a[i].first;
}
sort(a, a + n);
sort(b, b + n);
for(i = 0; i < n && a[i].second == b[i].second; ++i);
if(i == n) puts("YES");
else puts("NO");
}
return 0;
}