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

# Solution Notebook¶

## Problem: Sum of Two Integers.¶

See the LeetCode problem page.

## Constraints¶

• Can we assume we're working with 32 bit ints?
• Yes
• Can we assume the inputs are valid?
• No, check for None
• Can we assume this fits memory?
• Yes

## Test Cases¶

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


## Algorithm¶

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

a 0111
b 0101


First, add a and b, without worrying about the carry (0+0=0, 0+1=1, 1+1=0):

result = a ^ b = 0010

Next, calculate the carry (1+1=2). We'll need to left shift one to prepare for the next iteration when we move to the next most significant bit:

carry = (a&b) << 1 = 1010

If the carry is not zero, we'll need to add the carry to the result. Recursively call the function, passing in result and carry.

Below are the values of a, b, and the carry of a = 7 and b = 5, producing the result of 12.

a 0111
b 0101
-----
c 0101
a 0010
b 1010
-----
c 0010
a 1000
b 0100
-----
c 0000
a 1100
b 0000

c = carry = 0, return the result 1100


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


## Unit Test¶

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

class TestSumTwo(unittest.TestCase):

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

def main():
test = TestSumTwo()
test.test_sum_two()

if __name__ == '__main__':
main()

Overwriting test_sum_two.py

In [3]:
%run -i test_sum_two.py

Success: test_sum_two