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

Source: leetcode | Difficulty: MEDIUM_HARD
Tags: Tree algorithms

Sorting

Graph traversal

Starter code: # 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:
        

Solutions: