Given a complete binary tree, count the number of nodes.
Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is
completely filled, and all nodes in the last level are as far left as
possible. It can have between 1 and 2h nodes inclusive at the last level h.
[LeetCode] Count Complete Tree Nodes 求完全二叉树的节点个数
原文：http://www.cnblogs.com/grandyang/p/4567827.html