Python递归实现判断二叉树是否为平衡二叉树

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

def is_balanced(root):
    def helper(node):
        if not node:
            return 0
        left_height = helper(node.left)
        if left_height == -1:
            return -1
        right_height = helper(node.right)
        if right_height == -1:
            return -1
        if abs(left_height - right_height) > 1:
            return -1
        return max(left_height, right_height) + 1

    return helper(root) != -1