110. Balanced Binary Tree

Easy LeetCode

Given a binary tree, determine if it is height-balanced.

Example 1

Input: root = [3,9,20,null,null,15,7]

Output: true

Example 2

Input: root = [1,2,2,3,3,null,null,4,4]

Output: false

Example 3

Input: root = []

Output: true

Constraints

• The number of nodes in the tree is in the range [0, 5000]
• -104 <= Node.val <= 104

How to solve the problem

Code

• Recursion

# 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 isBalanced(self, root: Optional[TreeNode]) -> bool:
        def get_height(node) -> int:
            if node is None:
                return 0
            left_height = get_height(node.left) # determine left height
            if left_height == -1:
                return -1
            right_height = get_height(node.right)
            if right_height == -1 or abs(left_height - right_height) > 1:  # determine right height
                return -1
            return max(left_height, right_height) + 1 # max depth
        return get_height(root) != -1  # if the value returned from get_height is not -1, it is a balance tree

Complexity

• Time complexity: O(n), n == number of nodes
• Space complexity: O(n)