递归在Mergesort算法（Python）中如何工作？

Question

Here is my code (python 2.7):

   def merge_sort(a_list):
    print ("Splitting ", a_list)
    if len(a_list) > 1:
        mid = len(a_list) // 2
        left_half = a_list[:mid]
        #print left_half
        right_half = a_list[mid:]
        #print right_half
        merge_sort(left_half)
        merge_sort(right_half)

        i = 0
        j = 0
        k = 0


        print ("Left half is ", left_half)
        print ("Right half is ", right_half)
        while i < len(left_half) and j < len(right_half):

            if left_half[i] < right_half[j]:
                a_list[k] = left_half[i]
                i = i + 1
            else:
                a_list[k] = right_half[j]
                j = j + 1
            k = k + 1

        while i < len(left_half):
            a_list[k] = left_half[i]
            i = i + 1
            k = k + 1

        while j < len(right_half):
            a_list[k] = right_half[j]
            j = j + 1
            k = k + 1

    print("Merging ", a_list)



   a_list = [54, 26, 93, 17]
   merge_sort(a_list)
   print(a_list)

And my output is as follows:

`1.('Splitting ', [54, 26, 93, 17])
2.('Splitting ', [54, 26])
3.('Splitting ', [54])
4.('Merging ', [54])
5.('Splitting ', [26])
6.('Merging ', [26])
7.('Left half is ', [54])
8.('Right half is ', [26])
9.('Merging ', [26, 54])
10.('Splitting ', [93, 17])
11.('Splitting ', [93])
12.('Merging ', [93])
13.('Splitting ', [17])
14.('Merging ', [17])
15('Left half is ', [93])
16.('Right half is ', [17])
17.('Merging ', [17, 93])
18.('Left half is ', [26, 54])
19.('Right half is ', [17, 93])
20.('Merging ', [17, 26, 54, 93])
21.[17, 26, 54, 93]`

What I struggling with is that I don't know why in line 18 of the output, the left half suddenly becomes [26,54]. I know it is recursive, so should it be [54,26]? and so the right half should be [93, 17]?(line 19 output)

Does anyone has any idea? Thank you so much!

Answer 1

I know it is recursive, so should it be...

The fact that it's recursive doesn't affect the number ordering at all. The internal logic does

[54,26]?

Nope. Your logic ordered the numbers in ascending order

 # add the left half (the smaller number) to the merged list 
 if left_half[i] < right_half[j]:
     a_list[k] = left_half[i]
     i = i + 1
  else:  # the right half is smaller, so add it
      a_list[k] = right_half[j]
      j = j + 1

the left half suddenly becomes [26,54].

It's not suddenly. The recursive method call returned back up the call stack