input: Write a function to find a pair of numbers from the given list that add up to the given target. The function should return a list containing the pair if found, otherwise return None.
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 8)==[3, 5]
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 10)==[4, 6]
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 2)==[1, 1]
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 15)==None
  assert find_pair([], 10)==None
output: def find_pair(nums, target):
      seen = {}
      for num in nums:
          complement = target - num
          if complement in seen:
              return [complement, num]
          seen[num] = True
      return None