Leetcode 222. Count Complete Tree Nodes

Problem

Given the root of a complete binary tree, return the number of the nodes in the tree.

According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, 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.

Design an algorithm that runs in less than O(n) time complexity.

Algorithm

Recursive.

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: Optional[TreeNode]) -> int:
        def count(root):
            if not root:
                return 0
            L = 0
            if root.left:
                L = count(root.left)
            R = 0
            if root.right:
                R = count(root.right)
            return L + R + 1
        return count(root)
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