UVa10763 - Foreign Exchange

//超时的
#include<iostream>
#include<vector>
using namespace std;
vector<int> a;
vector<int> b;
int n,x,y;
int main()
{
    while( cin >> n && n ){
        int t = 1;
        for(int i = 0; i < n; i++){
            cin >> x >> y;
            a.push_back(x);
            b.push_back(y);
        }
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                if(a[i] == b[j] && a[j] ==b[i] && i != j)
                    a[i] = a[j] = b[i] = b[j] = 0;
            }
        }
        for(int i = 0; i < n; i++)
        if(a[i]) { t = 0; break; }
        if(t) cout << "YES\n";
        else cout << "NO\n";
        a.clear();
        b.clear();
    }
    return 0;
}

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
struct candidate{
    int a;
    int b;
};
vector<candidate>cd;
vector<candidate>excd;
int n;
candidate x;
bool compare(candidate x, candidate y){
    if(x.a != y.a) return x.a < y.a;
    else return x.b < y.b;
}
int main()
{
    while( cin >> n && n){
        int t = n;
        for(int i = 0; i < n; i++){
            cin >> x.a >> x.b;
            cd.push_back(x);
            swap(x.a, x.b);
            excd.push_back(x);
        }
        sort(cd.begin(), cd.end(), compare);
        sort(excd.begin(), excd.end(), compare);
        for(int i = 0; i < n; i++)
            if(cd[i].a == excd[i].a && cd[i].b) t--;
        if(t) cout << "NO" << endl;
        else cout << "YES" << endl;
        cd.clear();
        excd.clear();
    }
    return 0;
}