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

Solution Notebook

Problem: Implement breadth-first traversal on a binary tree.

Constraints

  • Can we assume we already have a Node class with an insert method?
    • Yes
  • Can we assume this fits in memory?
    • Yes
  • What should we do with each node when we process it?
    • Call an input method visit_func on the node

Test Cases

Breadth-First Traversal

  • 5, 2, 8, 1, 3 -> 5, 2, 8, 1, 3

Algorithm

  • Initialize queue with root
  • While queue is not empty
    • Dequeue and print the node
    • Queue the left child
    • Queue the right child

Complexity:

  • Time: O(n)
  • Space: O(n), extra space for the queue

Code

In [1]:
%run ../bst/bst.py
In [2]:
from collections import deque


class BstBfs(Bst):

    def bfs(self, visit_func):
        if self.root is None:
            raise TypeError('root is None')
        queue = deque()
        queue.append(self.root)
        while queue:
            node = queue.popleft()
            visit_func(node)
            if node.left is not None:
                queue.append(node.left)
            if node.right is not None:
                queue.append(node.right)

Unit Test

In [3]:
%run ../utils/results.py
In [4]:
%%writefile test_bfs.py
import unittest


class TestBfs(unittest.TestCase):

    def __init__(self, *args, **kwargs):
        super(TestBfs, self).__init__()
        self.results = Results()

    def test_bfs(self):
        bst = BstBfs(Node(5))
        bst.insert(2)
        bst.insert(8)
        bst.insert(1)
        bst.insert(3)
        bst.bfs(self.results.add_result)
        self.assertEqual(str(self.results), '[5, 2, 8, 1, 3]')

        print('Success: test_bfs')


def main():
    test = TestBfs()
    test.test_bfs()


if __name__ == '__main__':
    main()
Overwriting test_bfs.py
In [5]:
%run -i test_bfs.py
Success: test_bfs