input: Write a function to find a pair of numbers from the given list that add up to the given target. If such a pair exists, return the pair as a list. If not, return None.
  assert find_pair([1,2,3,4,4,5,6], 8)==[2, 6]
  assert find_pair([1,2,3,4,4,5,6], 9)==[3, 6]
  assert find_pair([1,2,3,4,4,5,6], 10)==None
  assert find_pair([1,2,3,4,4,5,6], 7)==[1, 6]
  assert find_pair([1,2,3,4,4,5,6], 12)==None
output: def find_pair(nums, target):
      seen = set()
      for num in nums:
          complement = target - num
          if complement in seen:
              return [complement, num]
          seen.add(num)
      return None