110. Balanced Binary Tree

# 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: TreeNode) -> bool:
        return self.maxDepth(root) != -1
    def maxDepth(self, root):
        if root == None:
            return 0
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)
        if left != -1 and right != -1 and abs(left-right) <= 1:
            return max(left, right) + 1
        else:
            return -1