Foreign Exchange#1226
Your non-profit organization coordinates a student exchange program and needs your help.
The exchange program works as follows: each participant specifies their current place of residence and the place to which they would like to move. The program is considered successful if a suitable exchange partner can be found for every student. In other words, if a student wants to move from to , there must exist another student who wants to move from to .
Such a check is easy to perform when there are only participants, but what should be done if there are ?
The first line contains the number of test cases . The first line of each test case contains the number of students n~(1 \le n \le 100001). This is followed by lines describing the exchange data.
Each of these lines contains information about one student: two integers representing the student's current place of residence and the desired destination. Locations are given as non-negative integers not exceeding 10^9. The current and desired locations are different for every student.
For each test case, print "YES" on a separate line if the exchange program can be carried out successfully, and "NO" otherwise.
Example 1