在Python的歸並排序中使用遞歸

Question

我正在學習python中的歸類排序，並且對以下程序的實現有些困惑。 根據我的理解，當mergeSort ([3,1], compare) ，它分為mergeSort([3], compare)和mergeSort([1], compare) 。 然后應調用merge([3],[1], compare) ，這樣print語句應為left = [1,3] ，但該程序實際上將輸出為left = [3,1] ，其他情況相同print報表，為什么？ 非常感謝你。

def merge(left, right, compare):
    """Assumes left and right are sorted lists and compare defines an ordering
       on the elements.
       Returns a new sorted (by compare list containing the same elements as
       (left + right) would contain."""
    result = []
    i,j = 0, 0
    while i < len(left) and j < len(right):
        if compare(left[i], right[j]):
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1
    while (i < len(left)):
        result.append(left[i])
        i += 1
    while (j < len(right)):
        result.append(right[j])
        j += 1
    return result

import operator

def mergeSort(L, compare = operator.lt):
    """Assumes L is a list, compare defines an ordering on elements of L
       Returns a new sorted list containing the same elements as L"""
    if len(L) < 2:
        return L[:]
    else:
        middle = len(L)//2
        print "middle =", middle
        left = mergeSort(L[:middle], compare)
        print "left = ", L[:middle]
        right = mergeSort(L[middle:], compare)
        print "right = ", L[middle:]
        return merge(left, right, compare)

---

>>> L = [3,1,2,5,4,9,6,7,8]
>>> mergeSort(L, compare = operator.lt)
middle = 4
middle = 2
middle = 1
left =  [3]
right =  [1]
left =  [3, 1]
middle = 1
left =  [2]
right =  [5]
right =  [2, 5]
left =  [3, 1, 2, 5]
middle = 2
middle = 1
left =  [4]
right =  [9]
left =  [4, 9]
middle = 1
left =  [6]
middle = 1
left =  [7]
right =  [8]
right =  [7, 8]
right =  [6, 7, 8]
right =  [4, 9, 6, 7, 8]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

Answer 1

這就是問題：

    print "left = ", L[:middle] # content of L before sorting

將此行和其他類似的print語句替換為：

    print "left = ", left # actual result of mergeSort

Answer 2

left = mergeSort(L[:middle], compare)
print "left = ", left
right = mergeSort(L[middle:], compare)
print "right = ", right

這應該工作。

Answer 3

該代碼將L遞歸地分開打印。 它實際上並不打印任何合並的結果。 您可以通過更改行來查看每次合並的結果

return merge(left, right, compare)

至

merged = merge(left, right, compare)
print "merged = ", merged
return merged