input: Write a python function to return the pair of numbers that add up to the given sum.
  assert two_num_sum([1,9,7,2,8, 3, 4], 10) == [7,3]
  assert two_num_sum([4,5,6,7,2,3], 16) == [4, 12]
  assert two_num_sum([18,21,5], 6) == None
output: def two_num_sum(numbers, target_sum):
      num_set = set()
      for num in numbers:
          complement = target_sum - num
          if complement in num_set:
              return [num, complement]
          num_set.add(num)
      return None