input: Write a function to find a pair of numbers from a given list that add up to a given target number. If a pair is found, return the pair as a list; otherwise, 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], 10)==[4, 6]
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 12)==None
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 1)==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