input: Write a python function to find pairs of numbers in a list that add up to a target value.
  assert two_sum([8, 3, 10, 5], 13) == [(8, 5)]
  assert two_sum([2, 4, 3, 7, 5], 9) == [(2, 7), (4, 5)]
  assert two_sum([1, 1, 1, 2], 3) == [(1, 2)]
output: def two_sum(nums, target):
      pairs = []
      seen = set()
      for num in nums:
          complement = target - num
          if complement in seen:
              pairs.append((complement, num))
          seen.add(num)
      return pairs