LeetCode 222. 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.


   / \
  2   3
 / \  /
4  5 6

Output: 6


 counts nodes recursively one by one.

Python 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: TreeNode) -> int:
        if not root:
            return 0
        return self.countNodes(root.left) + self.countNodes(root.right) + 1
  • Time Complexity: ~N
  • Space Complexity: ~d = log(N) to keep the recursion stack, where d is a tree depth.

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