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;
}
}