input: Write a function to check if any two numbers from a given list sum up to a specified target number and return True if such a pair is found, otherwise return False.
  assert check_sum([1, 2, 3, 4], 7)==True
  assert check_sum([1, 2, 3, 4], 8)==False
  assert check_sum([10, 15, 3, 7], 17)==True
  assert check_sum([10, 15, 3, 7], 20)==False
  assert check_sum([], 9)==False
output: def check_sum(nums, target):
      seen = set()
      for num in nums:
          if target - num in seen:
              return True
          seen.add(num)
      return False