input: Write a function to check if any two numbers in a list add up to a target value.
  assert checkSum([1, 2, 3, 4, 5], 9) == False
  assert checkSum([1, 2, 3, 4, 5], 8) == True
  assert checkSum([3, 2, 5, 7], 10) == True
output: def checkSum(lst, target):
      seen = set()
      for num in lst:
          if target - num in seen:
              return True
          seen.add(num)
      return False