Inspiration from @yuleleegerlee

```
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def countNodes(self, root):
"""
:type root: TreeNode
:rtype: int
"""
def query_depth(node):
h = 0
while node:
h += 1
node = node.right
return h
d = query_depth(root)
def count(node, depth):
if not node: return 0
if query_depth(node.left) == depth-1:
return 2**(depth - 2) + count(node.right, depth - 1)
return count(node.left, depth - 1)
return count(root, d+1) + 2**d-1
```