UVa 10763 - Foreign Exchange

#include <algorithm>
#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <cmath>

using namespace std;

typedef struct snode
{
int a,b,l,r;
}student;
student S[500001];
int     U[500001];

int cmp(student p, student q)
{
if (p.l == q.l)
return p.r < q.r;
return p.l < q.l;
}

int main()
{
int n,x,y;
while (~scanf("%d",&n) && n) {
for (int i = 0 ; i < n ; ++ i) {
scanf("%d%d",&x,&y);
S[i].a = x; S[i].b = y;
S[i].l = min(x, y);
S[i].r = max(x, y);
}

int flag = 1;
if (n%2 == 0) {
sort(S, S+n, cmp);
memset(U, 0, sizeof(U));
for (int k,j,i = 0 ; i < n ; ++ i)
if (!U[i]) {
U[i] = 1;
j = i+1; k = 0;
while (S[j].l == S[i].l && S[j].r == S[i].r) {
if (!U[j] && S[i].a == S[j].b && S[i].b == S[j].a) {
U[j] = 1;
k = 1;
break;
}
j ++;
}
if (!k) {
flag = 0;
break;
}
}
}else flag = 0;

if (flag) printf("YES\n");
else printf("NO\n");
}
return 0;
}