input: Write a function to determine if any two numbers in a given list add up to a specific given number. If such two numbers exist, return True; otherwise, return False. If the given list is empty, return False.
  assert two_sum([2, 7, 11, 15], 9)==True
  assert two_sum([3, 2, 4], 6)==True
  assert two_sum([3, 3], 6)==True
  assert two_sum([1, 2, 3, 4], 8)==False
  assert two_sum([], 5)==False
output: def two_sum(nums, target):
      seen = set()
      for num in nums:
          complement = target - num
          if complement in seen:
              return True
          seen.add(num)
      return False