count-complete-tree-nodes_222.py

# Definition for a binary tree node.
# 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.

# Example 1:


# Input: root = [1,2,3,4,5,6]
# Output: 6
# Example 2:

# Input: root = []
# Output: 0
# Example 3:

# Input: root = [1]
# Output: 1


# Constraints:

# The number of nodes in the tree is in the range [0, 5 * 104].
# 0 <= Node.val <= 5 * 104
# The tree is guaranteed to be complete.

# ---------------------------------------Runtime 64 ms Beats 26.80% Memory 21.87 MB Beats 65.76%---------------------------------------

from typing import Optional


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:
        self.counter = 0
        if root:

            def dfs(node: TreeNode) -> None:
                if node:
                    self.counter += 1
                    dfs(node.left)
                    dfs(node.right)

            dfs(root)
        return self.counter