"""

The tree is balanced if

1. Left child and right child are balanced

2. |left_max_depth - right_max_depth| must smaller or equal to 1

So the helper() will check condition 1. and 2. by `if l!=-1 and r!=-1 and abs(l-r)<=1`

if true, the node is balanced, return the depth

if not, the node is not balanced, return -1

"""

class Solution(object):

def isBalanced(self, root):

def helper(node, depth):

if not node.left and not node.right: return depth

l = r = depth #l: left_max_depth, r: right_max_depth

if node.left:

l = helper(node.left, depth+1)

if node.right:

r = helper(node.right, depth+1)

if l!=-1 and r!=-1 and abs(l-r)<=1:

return max(l, r)

return -1

if not root: return True

return helper(root, 0)!=-1