Count Complete Tree Nodes

Given a complete binary tree, count the number of nodes.

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.

Solution

# 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
        """
        if root is None:
            return 0
        hl, hr = 0, 0
        left, right = root, root
        while left:
            hl+=1
            left = left.left
        while right:
            hr+=1
            right = right.right
        if hl == hr:
            return 2 ** hl -1
        else:
            return 1 + self.countNodes(root.left) + self.countNodes(root.right)