uva10763

2004

　　题目就是求能否匹配所有交流生，让他们互相换。 。 一开始第一反应是训练指南上第一道勇士杀龙问题，利用排序比较。。虽然过了但是网上看到一个反例。。就是10 20,20 30,30 10，。。。 一个正确得方法是利用图的思想（有点类似跷跷板。具体看连接中博客吧）
　　题目：

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 n
lines 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

　　
　　
　　自己代码不贴了。。
　　http://blog.iyunv.com/actoy/article/details/8813366