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to Binary Search Tree in the Python Track

Published at Jul 21 2018 · 0 comments
Test suite


This exercise has changed since this solution was written.

Insert and search for numbers in a binary tree.

When we need to represent sorted data, an array does not make a good data structure.

Say we have the array [1, 3, 4, 5], and we add 2 to it so it becomes [1, 3, 4, 5, 2] now we must sort the entire array again! We can improve on this by realizing that we only need to make space for the new item [1, nil, 3, 4, 5], and then adding the item in the space we added. But this still requires us to shift many elements down by one.

Binary Search Trees, however, can operate on sorted data much more efficiently.

A binary search tree consists of a series of connected nodes. Each node contains a piece of data (e.g. the number 3), a variable named left, and a variable named right. The left and right variables point at nil, or other nodes. Since these other nodes in turn have other nodes beneath them, we say that the left and right variables are pointing at subtrees. All data in the left subtree is less than or equal to the current node's data, and all data in the right subtree is greater than the current node's data.

For example, if we had a node containing the data 4, and we added the data 2, our tree would look like this:


If we then added 6, it would look like this:

 / \
2   6

If we then added 3, it would look like this

 /   \
2     6

And if we then added 1, 5, and 7, it would look like this

    /   \
   /     \
  2       6
 / \     / \
1   3   5   7

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run the appropriate command below (why they are different):

  • Python 2.7: py.test binary_search_tree_test.py
  • Python 3.4+: pytest binary_search_tree_test.py

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): python -m pytest binary_search_tree_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/binary-search-tree directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see Running the Tests.


Josh Cheek https://twitter.com/josh_cheek

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import unittest

from binary_search_tree import BinarySearchTree, TreeNode

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.0.0

class BinarySearchTreeTest(unittest.TestCase):

    def test_data_is_retained(self):
        expected = TreeNode('4', None, None)
        self.assertTreeEqual(BinarySearchTree(['4']).data(), expected)

    # Test inserting data at proper node
    def test_smaller_data_at_left_node(self):
        expected = TreeNode('4', TreeNode('2', None, None), None)
        self.assertTreeEqual(BinarySearchTree(['4', '2']).data(), expected)

    def test_same_number_at_left_node(self):
        expected = TreeNode('4', TreeNode('4', None, None), None)
        self.assertTreeEqual(BinarySearchTree(['4', '4']).data(), expected)

    def test_greater_number_at_right_node(self):
        expected = TreeNode('4', None, TreeNode('5', None, None))
        self.assertTreeEqual(BinarySearchTree(['4', '5']).data(), expected)

    def test_can_create_complex_tree(self):
        expected = TreeNode(
                TreeNode('1', None, None),
                TreeNode('3', None, None)
                TreeNode('5', None, None),
                TreeNode('7', None, None)
            BinarySearchTree(['4', '2', '6', '1', '3', '5', '7']).data(),

    # Test can sort data
    def test_can_sort_single_number(self):
        self.assertEqual(BinarySearchTree(['2']).sorted_data(), ['2'])

    def test_can_sort_if_second_number_is_smaller_than_first(self):
            BinarySearchTree(['2', '1']).sorted_data(), ['1', '2']

    def test_can_sort_if_second_number_is_same_as_first(self):
            BinarySearchTree(['2', '2']).sorted_data(), ['2', '2']

    def test_can_sort_if_second_number_is_greater_than_first(self):
            BinarySearchTree(['2', '3']).sorted_data(), ['2', '3']

    def test_can_sort_complex_tree(self):
            BinarySearchTree(['2', '1', '3', '6', '7', '5']).sorted_data(),
            ['1', '2', '3', '5', '6', '7']

    # Utilities
    def assertTreeEqual(self, tree_one, tree_two):
            self.compare_tree(tree_one, tree_two)
        except AssertionError:
            raise AssertionError("{} != {}".format(tree_one, tree_two))

    def compare_tree(self, tree_one, tree_two):
        self.assertEqual(tree_one.data, tree_two.data)

        # Compare left tree nodes
        if tree_one.left and tree_two.left:
            self.compare_tree(tree_one.left, tree_two.left)
        elif tree_one.left is None and tree_two.left is None:
            raise AssertionError

        # Compare right tree nodes
        if tree_one.right and tree_two.right:
            self.compare_tree(tree_one.right, tree_two.right)
        elif tree_one.right is None and tree_two.right is None:
            raise AssertionError

if __name__ == '__main__':
class TreeNode(object):
    def __init__(self, data, left, right):
        self.data = data
        self.left = left
        self.right = right

    def __str__(self):
        fmt = 'TreeNode(data={}, left={}, right={})'
        return fmt.format(self.data, self.left, self.right)

class BinarySearchTree(object):

    def __init__(self, tree_data):
        self.root = TreeNode(tree_data.pop(0), None, None)

        while len(tree_data) > 0:
            ptr = self.root
            node = TreeNode(tree_data.pop(0), None, None)
            while (ptr != None):
                if ptr.data >= node.data:
                    next_ptr = ptr.left
                    if next_ptr is None:
                        ptr.left = node
                if ptr.data < node.data:
                    next_ptr = ptr.right
                    if next_ptr is None:
                        ptr.right = node
                ptr = next_ptr

    def data(self):
        return self.root

    def sorted_data(self):  # infix traversal
        return self._inorder_traversal(self.root)

    def _inorder_traversal(self, root):
        if root:
            left_subtree = self._inorder_traversal(root.left)
            right_subtree = self._inorder_traversal(root.right)
            return []

        return left_subtree + [root.data] + right_subtree

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