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.
Input: 1 / \ 2 3 / \ / 4 5 6 Output: 6
counts nodes recursively one by one.
# 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.