input: Write a function to check if any two numbers from a given list sum up to a target number. If such two numbers exist, return True, otherwise return False.
  assert check_sum([1, 2, 3, 4, 5], 9)==True
  assert check_sum([1, 2, 3, 4, 5], 10)==False
  assert check_sum([10, 20, 30, 40], 50)==True
  assert check_sum([10, 20, 30, 40], 100)==False
  assert check_sum([], 0)==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