new_contents: #!/path/to/python
  
  def merge_sort(items):
      if items is None:
          return
  
      helper = []
      merge_sort_impl(items, helper, 0, len(items))
  
  def merge_sort_impl(items, helper, start, end):
      if (end - start) <= 1:
          return
      mid = (end - start) / 2 + start
      merge_sort_impl(items, helper, start, mid)
      merge_sort_impl(items, helper, mid, end)
      merge(items, helper, start, mid, end)
  
  def merge(items, helper, start, mid, end):
      leftStart = start
      leftEnd = mid
      rightStart = mid
      rightEnd = end
  
      while leftStart < leftEnd and rightStart < rightEnd:
          if items[leftStart] < items[rightStart]:
              helper.append(items[leftStart])
              leftStart += 1
          else:
              helper.append(items[rightStart])
              rightStart += 1
  
      while leftStart < leftEnd:
          helper.append(items[leftStart])
          leftStart += 1
      while rightStart < rightEnd:
          helper.append(items[rightStart])
          rightStart += 1
  
      cnt = len(helper)
      for i in range(0, cnt):
          items[start + i] = helper[i]
  
      del helper[:]
  
subject: Add sort code.