balanced-binary-tree.py

"""
https://leetcode.com/submissions/detail/147673610/

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 depth of the two subtrees of every node never differ by more than 1.


Example 1:

Given the following tree [3,9,20,null,null,15,7]:


    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:


       1
      / \
     2   2
    / \
   3   3
  / \
 4   4


Return false.
"""


# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def isBalanced(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        def walk(node):
            if not node:
                return 0
            left = walk(node.left)
            right = walk(node.right)
            if left == -1 or right == -1 or abs(left - right) > 1:
                return -1
            return 1 + max(left, right)

        return walk(root) != -1