Interviewcake problem: is a tree "superbalanced"? https://www.interviewcake.com/question/python/balanced-binary-tree

superbalanced.py

#!/usr/bin/env python

class BinaryTreeNode(object):

    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None

    def insert_left(self, value):
        self.left = BinaryTreeNode(value)
        return self.left

    def insert_right(self, value):
        self.right = BinaryTreeNode(value)
        return self.right

def difference(a, b):
    return abs(a - b)

def depths(tree, depth=1):
    if tree.left and tree.right:
        # we have both right and left children
        (lmin, lmax) = depths(tree.left, depth+1)
        (rmin, rmax) = depths(tree.right, depth+1)
        return (min(lmin, rmin), max(lmax, rmax))
    elif tree.left:
        # we only have a left child
        return depths(tree.left, depth+1)
    elif tree.right:
        # we only have a right child
        return depths(tree.right, depth+1)
    else:
        # left and right are both None
        return (depth, depth)

def is_superbalanced(tree):
    (min_depth, max_depth) = depths(tree)
    return difference(min_depth, max_depth) <= 1

def test1():
    # tree (is superbalanced):
    # o
    # 
    root = BinaryTreeNode(None)
    assert is_superbalanced(root) == True

def test2():
    # tree (is superbalanced):
    #    o
    #   /
    #  o
    root = BinaryTreeNode(None)
    root.insert_left(None)
    assert is_superbalanced(root) == True

def test3():
    # tree (is superbalanced):
    #    o
    #   / \
    #  o   o
    root = BinaryTreeNode(None)
    root.insert_left(None)
    root.insert_right(None)
    assert is_superbalanced(root) == True

def test4():
    # tree (is superbalanced):
    #      o
    #     /
    #    o
    #   /
    #  o
    root = BinaryTreeNode(None)
    child = root.insert_left(None)
    child = child.insert_left(None)
    assert is_superbalanced(root) == True

def test5():
    # tree (is superbalanced):
    #      o
    #     / \
    #    o   o
    #   /
    #  o
    root = BinaryTreeNode(None)
    child = root.insert_left(None)
    child = child.insert_left(None)
    child = root.insert_right(None)
    assert is_superbalanced(root) == True

def test6():
    # tree (is NOT superbalanced):
    #        o
    #       / \
    #      o   o
    #     /
    #    o
    #   /
    #  o
    root = BinaryTreeNode(None)
    child = root.insert_left(None)
    child = child.insert_left(None)
    child = child.insert_left(None)
    child = root.insert_right(None)
    assert is_superbalanced(root) == False

if __name__ == "__main__":
    test1()
    test2()
    test3()
    test4()
    test5()
    test6()