# Question

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


# Solution

# 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
"""
return not (self.height(root) == -1)

def height(self, root):
"""
:type root: TreeNode
:rtype: int, -1 if not balanced
"""
if root is None:
return 0
else:
left = self.height(root.left)
if left == -1:
return -1
right = self.height(root.right)
if right == -1:
return -1

if -1 <= left - right <= 1:
return max(left, right) + 1
else:
return -1