Given a complete binary tree, count the number of nodes.

Note:
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.

Example:

Input: 
    1
   / \
  2   3
 / \  /
4  5 6

Output: 6

难度：medium

题目：给定完全二叉树，统计其结点数。

思路：后序遍历

Runtime: 1 ms, faster than 100.00% of Java online submissions for Count Complete Tree Nodes.
Memory Usage: 40.4 MB, less than 45.43% of Java online submissions for Count Complete Tree Nodes.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int countNodes(TreeNode root) {
        if (null == root) {
            return 0;
        }
        
        return countNodes(root.left) + countNodes(root.right) + 1;
    }
}