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.

解题要点：

1.利用之前判定二叉树最大深度的函数，左右子树层数相减不大于1即为平衡树；

# 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
        """
        if root == None:
            return True
        elif abs(self.height(root.left)-self.height(root.right))>1:
            return False
        else:
            return self.isBalanced(root.left) and self.isBalanced(root.right)
    
    def height(self,root):
        if root == None:
            return 0
        else:
            return max(self.height(root.left),self.height(root.right))+1