input: Write a function to find a pair of numbers from a given list that add up to a specified target number. If such a pair exists, return the pair as a list. If no such pair exists, return an empty list.
  assert find_pair([1, 2, 3, 4, 4, 5, 6], 8)==[4, 4]
  assert find_pair([1, 2, 3, 4, 5], 7)==[2, 5]
  assert find_pair([1, 2, 3, 4, 5], 10)==[]
  assert find_pair([], 5)==[]
  assert find_pair([5], 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 []