Problem

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

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 isBalanced(self, root: Optional[TreeNode]) -> bool:
        def check(root):
            if root is None:
                return 0
            
            # exhaust both left and right trees
            left = check(root.left)
            right = check(root.right)

            # check if our trees are unbalanced
            # we use abs because one side might be more than the other
            # resulting in a negative number
            if left == -1 or right == -1 or abs(left - right) > 1:
                return -1
            # Increment by one since we have counted a node
            return 1 + max(left, right)
        return check(root) != -1