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

Solution Notebook

Problem: Find the difference of two integers without using the + or - sign.

Constraints

  • Can we assume the inputs are valid?
    • No, check for None
  • Can we assume this fits memory?
    • Yes

Test Cases

* None input -> TypeError
* 7, 5 -> 2
* -5, -7 -> 2
* -5, 7 -> -12
* 5, -7 -> 12

Algorithm

We'll look at the following example, subtracting a and b:

a 0110 = 6 
b 0101 = 5

First, subtract a and b, without worrying about the borrow (0-0=0, 0-1=1, 1-1=0):

result = a ^ b = 0011

Next, calculate the borrow (0-1=1). We'll need to left shift one to prepare for the next iteration when we move to the next most significant bit:

~a = 1001 b = 0101 ~a & b = 0001

borrow = (~a&b) << 1 = 0010

If the borrow is not zero, we'll need to subtract the borrow from the result. Recursively call the function, passing in result and borrow.

Complexity:

  • Time: O(b), where b is the number of bits
  • Space: O(b), where b is the number of bits

Code

In [1]:
class Solution(object):

    def sub_two(self, a, b):
        if a is None or b is None:
            raise TypeError('a or b cannot be None')
        result = a ^ b;
        borrow = (~a & b) << 1
        if borrow != 0:
            return self.sub_two(result, borrow)
        return result;

Unit Test

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


class TestSubTwo(unittest.TestCase):

    def test_sub_two(self):
        solution = Solution()
        self.assertRaises(TypeError, solution.sub_two, None)
        self.assertEqual(solution.sub_two(7, 5), 2)
        self.assertEqual(solution.sub_two(-5, -7), 2)
        self.assertEqual(solution.sub_two(-5, 7), -12)
        self.assertEqual(solution.sub_two(5, -7), 12)
        print('Success: test_sub_two')


def main():
    test = TestSubTwo()
    test.test_sub_two()


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