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 6Output: 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;    }}