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)