input: Write a function to find a pair of numbers from a given list that add up to a given target number. If such a pair is found, return the pair as a list. Otherwise, return an empty list.
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 8)==[4, 4]
  assert find_pair([1, 9, 3, 7, 5, 6], 14)==[9, 5]
  assert find_pair([10, 20, 30, 40], 60)==[20, 40]
  assert find_pair([1, 2, 3, 4, 5], 10)==[]
  assert find_pair([], 5)==[]
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 []