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

Challenge Notebook

Problem: Given an array, find the two indices that sum to a specific value.

Constraints

  • Is there exactly one solution?
    • Yes
  • Is there always a solution?
    • Yes
  • Is the array an array of ints?
    • Yes
  • Is the array sorted? No
  • Are negative values possible?
    • Yes
  • Can we assume the inputs are valid?
    • No
  • Can we assume this fits memory?
    • Yes

Test Cases

  • None input -> TypeError
  • [] -> ValueError
  • [1, 3, 2, -7, 5], 7 -> [2, 4]

Algorithm

Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

Code

In [ ]:
class Solution(object):

    def two_sum(self, nums, val):
        # TODO: Implement me
        pass

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load test_two_sum.py
import unittest


class TestTwoSum(unittest.TestCase):

    def test_two_sum(self):
        solution = Solution()
        self.assertRaises(TypeError, solution.two_sum, None, None)
        self.assertRaises(ValueError, solution.two_sum, [], 0)
        target = 7
        nums = [1, 3, 2, -7, 5]
        expected = [2, 4]
        self.assertEqual(solution.two_sum(nums, target), expected)
        print('Success: test_two_sum')


def main():
    test = TestTwoSum()
    test.test_two_sum()


if __name__ == '__main__':
    main()

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.