題意:有一個交換生由A->B,想交換得有一個B->A，問,是不是所有人都能交換成.

倆個數字交換偶數次還是自身,開一個數組mark,模擬完所有樣例後,看陣列是不是還是初始化陣列.

#include "pch.h"
#include <string>
#include<iostream>
#include<map>
#include<memory.h>
#include<vector>
#include<algorithm>
#include<queue>
#include<vector>


namespace cc
{
	using std::cout;
	using std::endl;
	using std::cin;
	using std::map;
	using std::vector;
	using std::string;
	using std::sort;
	using std::priority_queue;
	using std::greater;
	using std::vector;
	using std::swap;

	constexpr int N = 500001;
	//constexpr int N = 30;

	//priority_queue<int,vector<int>, greater<int> >q;

	int mark[N];
	void init()
	{
		for (int i = 0;i < N;i++)
			mark[i] = i;
	}

	void solve()
	{
		int n, s, e;
		while (cin >> n && n)
		{
			init();
			for (int i = 0;i < n;i++)
			{
				cin >> s >> e;
				auto t = mark[e];
				mark[e] = mark[s];
				mark[s] = t;
			}
			int ok = 1;
			for (int i = 0;i < N;i++)
				if (mark[i] != i)
				{
					ok = 0;
					break;
				}
			if (ok)
				cout << "YES" << endl;
			else
				cout << "NO" << endl;
		}


	}

};


int main()
{

#ifndef ONLINE_JUDGE
	freopen("d://1.text", "r", stdin);
#endif // !ONLINE_JUDGE
	cc::solve();

	return 0;
}