input: Write a function to optimize the following Python code that sorts a list of numbers in O(n) time complexity. The input is an unsorted list, and the output should be the sorted list.
  assert sort_list([2, 6, 3, 8, 4])==[2, 3, 4, 6, 8]
  assert sort_list([5, 1, 9, 7, 3])==[1, 3, 5, 7, 9]
  assert sort_list([10, 10, 10])==[10, 10, 10]
  assert sort_list([])==[]
  assert sort_list([1])==[1]
  assert sort_list([4, 4, 2, 2])==[2, 2, 4, 4]
output: def sort_list(nums):
      if not nums:
          return []
      max_num = max(nums)
      count = [0] * (max_num + 1)
      for num in nums:
          count[num] += 1
      sorted_list = []
      for i in range(max_num + 1):
          sorted_list.extend([i] * count[i])
      return sorted_list