This notebook was prepared by Donne Martin. Source and license info is on GitHub.

Challenge Notebook

Constraints

  • What does it mean to invert a binary tree?
    • Swap all left and right node pairs
  • Can we assume we already have a Node class?
    • Yes
  • Can we assume the inputs are valid?
    • No
  • Can we assume this fits memory?
    • Yes

Test Cases

Input:
     5
   /   \
  2     7
 / \   / \
1   3 6   9

Output:
     5
   /   \
  7     2
 / \   / \
9   6 3   1

Algorithm

Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

Code

In [ ]:
%run ../bst/bst.py
In [ ]:
class InverseBst(Bst):

    def invert_tree(self):
        # TODO: Implement me
        pass

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load test_invert_tree.py
import unittest


class TestInvertTree(unittest.TestCase):

    def test_invert_tree(self):
        root = Node(5)
        bst = InverseBst(root)
        node2 = bst.insert(2)
        node3 = bst.insert(3)
        node1 = bst.insert(1)
        node7 = bst.insert(7)
        node6 = bst.insert(6)
        node9 = bst.insert(9)
        result = bst.invert_tree()
        self.assertEqual(result, root)
        self.assertEqual(result.left, node7)
        self.assertEqual(result.right, node2)
        self.assertEqual(result.left.left, node9)
        self.assertEqual(result.left.right, node6)
        self.assertEqual(result.right.left, node3)
        self.assertEqual(result.right.right, node1)
        print('Success: test_invert_tree')


def main():
    test = TestInvertTree()
    test.test_invert_tree()


if __name__ == '__main__':
    main()

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.