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

Solution Notebook

Problem: Move all zeroes in a list to the end.

Constraints

  • Is the input an array of ints?
    • Yes
  • Is the output a new array of ints?
    • No, do this in-place
  • Do we need to maintain ordering of non-zero values?
    • Yes
  • Can we assume the inputs are valid?
    • No
  • Can we assume this fits memory?
    • Yes

Test Cases

* None -> TypeError
* [0, 1, 0, 3, 12] -> [1, 3, 12, 0, 0]
* [1, 0] -> [1, 0]
* [0, 1] -> [1, 0]
* [0] -> [0]
* [1] -> [1]
* [1, 1] -> [1, 1]

Algorithm

  • pos = 0
  • Loop through each item in the input
    • If the item != 0, set input[pos] = item
      • pos++
  • Fill input[pos:] with zeroes
 |
[0, 1, 0, 3, 12]
 ^
    |
[0, 1, 0, 3, 12]
 ^
    |
[1, 1, 0, 3, 12]
 ^
       |
[1, 1, 0, 3, 12]
    ^
          |
[1, 1, 0, 3, 12]
    ^
          |
[1, 3, 0, 3, 12]
    ^
              |
[1, 3, 0, 3, 12]
       ^
              |
[1, 3, 12, 3, 12]
       ^

Fill right with zeroes:

[1, 3, 12, 3, 12]
           ^
[1, 3, 12, 0, 12]
           ^
[1, 3, 12, 0, 0]
              ^

Complexity:

  • Time: O(n)
  • Space: O(1)

Code

In [1]:
class Solution(object):

    def move_zeroes(self, nums):
        if nums is None:
            raise TypeError('nums cannot be None')
        pos = 0
        for num in nums:
            if num != 0:
                nums[pos] = num
                pos += 1
        if pos < len(nums):
            nums[pos:] = [0] * (len(nums) - pos)

Unit Test

In [2]:
%%writefile test_move_zeroes.py
import unittest


class TestMoveZeroes(unittest.TestCase):

    def test_move_zeroes(self):
        solution = Solution()
        self.assertRaises(TypeError, solution.move_zeroes, None)
        array = [0, 1, 0, 3, 12]
        solution.move_zeroes(array)
        self.assertEqual(array, [1, 3, 12, 0, 0])
        array = [1, 0]
        solution.move_zeroes(array)
        self.assertEqual(array, [1, 0])
        array = [0, 1]
        solution.move_zeroes(array)
        self.assertEqual(array, [1, 0])
        array = [0]
        solution.move_zeroes(array)
        self.assertEqual(array, [0])
        array = [1]
        solution.move_zeroes(array)
        self.assertEqual(array, [1])
        array = [1, 1]
        solution.move_zeroes(array)
        self.assertEqual(array, [1, 1])
        print('Success: test_move_zeroes')


def main():
    test = TestMoveZeroes()
    test.test_move_zeroes()


if __name__ == '__main__':
    main()
Overwriting test_move_zeroes.py
In [3]:
%run -i test_move_zeroes.py
Success: test_move_zeroes