222.Count_Complete_Tree_Nodes(Easy).md

222. Count Complete Tree Nodes (Easy)

Date and Time: Jul 16, 2024, 20:36 (EST)

Link: https://leetcode.com/problems/count-complete-tree-nodes/

Question:

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 2^h 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 * 10^4].

• 0 <= Node.val <= 5 * 10^4

• The tree is guaranteed to be complete.

KeyPoints:

My Solution:

# 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:
        if not root:
            return 0
        return 1 + self.countNodes(root.left) + self.countNodes(root.right)

Time Complexity: $O(n)$
Space Complexity: $O(1)$

Optimized Solution:

Time Complexity: $O(n)$
Space Complexity: $O()$