[排序]Foreign Exchange uva10763

Problem E
Foreign Exchange
Input: standard input
Output: standard output
Time Limit: 1 second

Your non-profit organization (iCORE - international Confederation of Revolver Enthusiasts) coordinates a very successful foreign student exchange program. Over the last few years, demand has sky-rocketed and now you need assistance with your task.

The program your organization runs works as follows: All candidates are asked for their original location and the location they would like to go to. The program works out only if every student has a suitable exchange partner. In other words, if a student wants to go from A to B, there must be another student who wants to go from B to A. This was an easy task when there were only about 50 candidates, however now there are up to 500000 candidates!

Input

The input file contains multiple cases. Each test case will consist of a line containing n - the number of candidates (1≤n≤500000), followed by nlines representing the exchange information for each candidate. Each of these lines will contain 2 integers, separated by a single space, representing the candidate's original location and the candidate's target location respectively. Locations will be represented by nonnegative integer numbers. You may assume that no candidate will have his or her original location being the same as his or her target location as this would fall into the domestic exchange program. The input is terminated by a case where n = 0; this case should not be processed.

 

Output

For each test case, print "YES" on a single line if there is a way for the exchange program to work out, otherwise print "NO".

 

Sample Input                               Output for Sample Input

10 1 2 2 1 3 4 4 3 100 200 200 100 57 2 2 57 1 2 2 1 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0

YES NO

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Problem setter: Gilbert Lee, University of Alberta, Canada

题意:

有一个非营利性的国际性机构执行一项国际交换学生计画。这几年来执行得相当好，有越来越多的学生想要参与这项计画，所以现在请你来帮忙这个机构。每个提出交换学生申请的人有2项资料：他来自哪一国，他想要交换到哪一国。这个计画只有在每个学生都可以找到一个交换对象的时候才算成功。换句话说，假如有一个学生想要从A国到B国，那么一定要有另一个学生从B国到A国。如果只有50个学生提出申请，要判断这计画是否可行相当简单，然而现在最多已经有 500000个学生了。

思路：这道题目的思路是求每个学生是否存在与之配对的交换学生。我的思路是把每个交换学生的信息拆分为两部分分别做下标记，然后排序，看要互相交换的学生数量是否相等