[222][Easy] Count Complete Tree Nodes.py

# 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):
        if root == None: return 0

        left = right = root
        l = r = 1
        while left  := left.left:   l += 1
        while right := right.right: r += 1
        
        if l == r: return 2**l - 1
        else: return 1 + self.countNodes(root.left) + self.countNodes(root.right)