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222. Count Complete Tree Nodes

Binary Search Bit Manipulation Tree Binary Tree

Problem - Count Complete Tree Nodes

Easy

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.

Solutions

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# 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 depth(root):
            dep = 0
            while root:
                dep += 1
                root = root.left
            return dep

        if root == None:
            return 0

        left, right = depth(root.left), depth(root.right)

        if left == right:
            return (1 << left) + self.countNodes(root.right)
        else:
            return (1 << right) + self.countNodes(root.left)

Submission Stats:

  • Runtime: 0 ms (100.00%)
  • Memory: 23.3 MB (82.15%)