## Description

https://leetcode.com/problems/count-complete-tree-nodes/

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 2^{h} nodes inclusive at the last level h.

**Example:**

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

## Explanation

counts nodes recursively one by one.

## Python Solution

```
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def countNodes(self, root: TreeNode) -> int:
if not root:
return 0
return self.countNodes(root.left) + self.countNodes(root.right) + 1
```

- Time Complexity: ~N
- Space Complexity: ~d = log(N) to keep the recursion stack, where d is a tree depth.