# LeetCode 222. Count Complete Tree Nodes

## 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 2h nodes inclusive at the last level h.

Example:

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

Output: 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.