import unittest
from collections import deque


def is_balanced(node):
    q, seen, ds = deque(), set(), set()
    q.append((0, node))
    while q:
        d, node = q.popleft()
        l, r = node.left, node.right
        seen.add(node)
        if not l and not r:
            if d not in ds and len(ds) == 2:
                return False
            else:
                ds.add(d)
        if l and l not in seen:
            q.append((d + 1, l))
        if r and r not in seen:
            q.append((d + 1, r))
    return max(ds) - min(ds) <= 1


# Tests
class Test(unittest.TestCase):
    class BinaryTreeNode(object):
        def __init__(self, value):
            self.value = value
            self.left = None
            self.right = None

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

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

    def test_full_tree(self):
        tree = Test.BinaryTreeNode(5)
        left = tree.insert_left(8)
        right = tree.insert_right(6)
        left.insert_left(1)
        left.insert_right(2)
        right.insert_left(3)
        right.insert_right(4)
        result = is_balanced(tree)
        self.assertTrue(result)

    def test_both_leaves_at_the_same_depth(self):
        tree = Test.BinaryTreeNode(3)
        left = tree.insert_left(4)
        right = tree.insert_right(2)
        left.insert_left(1)
        right.insert_right(9)
        result = is_balanced(tree)
        self.assertTrue(result)

    def test_leaf_heights_differ_by_one(self):
        tree = Test.BinaryTreeNode(6)
        left = tree.insert_left(1)
        right = tree.insert_right(0)
        right.insert_right(7)
        result = is_balanced(tree)
        self.assertTrue(result)

    def test_leaf_heights_differ_by_two(self):
        tree = Test.BinaryTreeNode(6)
        left = tree.insert_left(1)
        right = tree.insert_right(0)
        right_right = right.insert_right(7)
        right_right.insert_right(8)
        result = is_balanced(tree)
        self.assertFalse(result)

    def test_three_leaves_total(self):
        tree = Test.BinaryTreeNode(1)
        left = tree.insert_left(5)
        right = tree.insert_right(9)
        right.insert_left(8)
        right.insert_right(5)
        result = is_balanced(tree)
        self.assertTrue(result)

    def test_both_subtrees_superbalanced(self):
        tree = Test.BinaryTreeNode(1)
        left = tree.insert_left(5)
        right = tree.insert_right(9)
        right_left = right.insert_left(8)
        right.insert_right(5)
        right_left.insert_left(7)
        result = is_balanced(tree)
        self.assertFalse(result)

    def test_both_subtrees_superbalanced_two(self):
        tree = Test.BinaryTreeNode(1)
        left = tree.insert_left(2)
        right = tree.insert_right(4)
        left.insert_left(3)
        left_right = left.insert_right(7)
        left_right.insert_right(8)
        right_right = right.insert_right(5)
        right_right_right = right_right.insert_right(6)
        right_right_right.insert_right(9)
        result = is_balanced(tree)
        self.assertFalse(result)

    def test_only_one_node(self):
        tree = Test.BinaryTreeNode(1)
        result = is_balanced(tree)
        self.assertTrue(result)

    def test_linked_list_tree(self):
        tree = Test.BinaryTreeNode(1)
        right = tree.insert_right(2)
        right_right = right.insert_right(3)
        right_right.insert_right(4)
        result = is_balanced(tree)
        self.assertTrue(result)


unittest.main(verbosity=2)