归并算法——拆分
拆到每一个子部分只有一个元素
然后再进行合并
合并的时候，小的在前，大的在后

合并

当合并的时候，遇到两组元素合并，每组各有两个元素，需要建立游标，一左一右
思路：先把一个序列从头开始往下拆，一直拆到只有一个元素为止。
逐次合并

代码实现：

# coding:utf-8


def merge_sort(alist):
  """归并排序"""
  n = len(alist)
  if n <= 1:
      return alist
  mid = n//2

  # left 采用归并排序后形成的有序的新的列表
  left_li = merge_sort(alist[:mid])

  # right 采用归并排序后形成的有序的新的列表
  right_li = merge_sort(alist[mid:])

  # 将两个有序的子序列合并为一个新的整体
  # merge(left, right)
  left_pointer, right_pointer = 0, 0
  result = []

  while left_pointer < len(left_li) and right_pointer < len(right_li):
      if left_li[left_pointer] <=  right_li[right_pointer]:
          result.append(left_li[left_pointer])
          left_pointer += 1
      else:
          result.append(right_li[right_pointer])
          right_pointer += 1

  result += left_li[left_pointer:]
  result += right_li[right_pointer:]
  return result


if __name__ == "__main__":
  li = [54, 26, 93, 17, 77, 31, 44, 55, 20]
  print(li)
  sorted_li = merge_sort(li)
  print(li)
  print(sorted_li)

  # merge_sort   [54, 26, 93, 17, 77, 31, 44, 55, 20]
  #
  # left_li = merge_sort [54, 26, 93, 17]
  #
  #     left_li = merge_sort [54, 26]
  #     left_li = [26, 54]
  #
  #
  #              left_li = [54]
  #              right_li = [26]
  #              result = [26, 54]
  #              return result
  #
  #     right_li = merge_sort([93, 17])
  #
  #             left_li = merge_sort([93])
  #
  #                         return [93]
  #             left_li =[93]
  #
  #             right_li = merge_sort([17])
  #
  #                         return [17]
  #             right_li = [17]
  #
  #             result = [17, 93]
  #
  #             return result
  #
  #     right_li = [17, 93]
  #
  #     result = [17, 26, 54, 93]
  #
  #     return result
  #
  # left_li = [17, 26, 54, 93]
  #
  # right_li = merge_sort([77, 31, 44, 55, 20])
  #
  #
  # result = []
  # return result