python实现归并排序算法

#归并排序
 
def merge_sort(alist):
  if len(alist) <= 1:
    return alist
  # 二分分解
  num = len(alist) / 2
  left = merge_sort(alist[:num])
  right = merge_sort(alist[num:])
  # 合并
  return merge(left, right)
 
 
def merge(left, right):
  \'\'\'合并操作，将两个有序数组left[]和right[]合并成一个大的有序数组\'\'\'
  # left与right的下标指针
  l, r = 0, 0
  result = []
  while l < len(left) and r < len(right):
    if left[l] < right[r]:
      result.append(left[l])
      l += 1
    else:
      result.append(right[r])
      r += 1
  result += left[l:]
  result += right[r:]
  return result
 
 
alist = [54, 26, 93, 17, 77, 31, 44, 55, 20]
sorted_alist = merge_sort(alist)
print(sorted_alist)