[英]How does the recursive work in the Mergesort algorithms (Python)?
這是我的代碼(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)
我的輸出如下:
`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]`
我苦苦掙扎的是,我不知道為什么在輸出的第18行中,左半部分突然變成[26,54]。 我知道它是遞歸的,應該是[54,26]嗎? 所以右半部分應該是[93,17]?(第19行的輸出)
有人知道嗎? 非常感謝!
我知道它是遞歸的,所以應該...
它是遞歸的事實根本不會影響數字順序。 內部邏輯
[54,26]?
不。 您的邏輯按升序排列數字
# 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
左半部分突然變成[26,54]。
不是突然的。 遞歸方法調用返回了備份堆棧
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