input: Write a function to find the closest pair of numbers in an array.
  assert find_closest_pair([3, 5, 8, 7, 1, 12]) == [1, 3]
  assert find_closest_pair([1, 4, 7, 10, 13]) == [1, 4]
  assert find_closest_pair([10, 22, 28, 29, 30]) == [28, 29]
output: def find_closest_pair(arr):
      arr.sort()
      min_diff = float('inf')
      closest_pair = []
      for i in range(len(arr) - 1):
          diff = abs(arr[i] - arr[i + 1])
          if diff < min_diff:
              min_diff = diff
              closest_pair = [arr[i], arr[i + 1]]
      return closest_pair