input: Write a function to sort an array in ascending order. If the array is empty then return an empty array.
  def sort_ascending(array):
    if not array:
      return []
    array.sort()
    return array
  
  assert sort_ascending([10, 3, 9, 1, 15, 2])==[1, 2, 3, 9, 10, 15]
  assert sort_ascending([])==[]
output: def sort_ascending(array):
    if not array:
      return []
    array.sort()
    return array