Game

  Flynn and oooccc1 are playing game some day. The game is played on a rooted graph which is an undirected graph with every edge attached by some path to a special vertex called the root or the ground. The ground is denoted in the figure that follows by a dotted line. You could see it from the figure.



  On each step, one player could chop out one edge, and removing that edge and anything not connected to the ground. The one who have nothing to chop with fails.
  Flynn is the one who is going to move. Help him by telling him if it is possible to win by carefully choose the edge to chop with.

There are a few cases. Process to the end of file
Each case starts with N (1 <= N <= 1000) stand for the number of points.
The next line give N numbers, the i-th (i = 0 … N-1) of which represents its father nodes. The root nodes was the one with value -1.

Print “YES” if could win if play optimally, and “NO” otherwise.

8
-1 1 2 2 -1 5 6 6

NO

ZSTU

HDU 2009-12 Programming Contest