balanced_binary_tree.py

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

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the left and right subtrees of every node differ in height by no more than 1.
'''


# 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 height(root):
            if not root:
                return 0
            return 1 + max(height(root.left), height(root.right))


        if not root:
            return True
        
        if abs(height(root.left) - height(root.right)) > 1:
            return False
        
        return self.isBalanced(root.left) and self.isBalanced(root.right)
    
         
class Solution(object):
    def isBalanced(self, root):
        stack, node, last, depths = [], root, None, {}
        while stack or node:
            if node:
                stack.append(node)
                node = node.left
            else:
                node = stack[-1]
                if not node.right or last == node.right:
                    node = stack.pop()
                    left, right  = depths.get(node.left, 0), depths.get(node.right, 0)
                    if abs(left - right) > 1: return False
                    depths[node] = 1 + max(left, right)
                    last = node
                    node = None
                else:
                    node = node.right
        return True