計算一個tuple里面的逆序數(shù)，用merge sort的辦法。我寫了以下代碼，但是每次統(tǒng)計的時候，count設(shè)置為全局變量了：

'''Count inversion

Input: a sequence as tuple of integers

Output: The inversion number as an integer'''

#Merge Sort Method

def merge(ListA, ListB):

"""key subroutine

function: merge 2 sorted lists"""

newlist = []

while ListA and ListB:

if ListA[0] > ListB[0]:

newlist.append(ListB[0])

ListB = ListB[1:]

global count

count += len(ListA)

else:

newlist.append(ListA[0])

ListA = ListA[1:]

if ListA:

newlist = newlist + ListA

elif ListB:

newlist = newlist + ListB

return newlist

def merge_sort(A):

"""input: A(a list) the length of A can be odd

output: sorted list

"""

#base case: If n = 1, done

if len(A) == 1:

return A

#recursion: divide into two parts

else:#A[1, (n/2)] A[(n/2)+1, n] merge 2 lists

middle = len(A)/2 #if len(A) is odd number, in Python it will return the lower int of the result

A = merge(merge_sort(A[:middle]), merge_sort(A[middle:]))

return A

count = 0

def count_inversion(sequence):

sequence = list(sequence)

merge_sort(sequence)

return count

print count_inversion((1, 2, 5, 3, 4, 7, 6))

print count_inversion((0, 1, 2, 3))

print count_inversion((99, -99))

print count_inversion((5, 3, 2, 1, 0))

如果只運(yùn)行一次，那么這個程序是對的。但是如果要運(yùn)行好幾次，由于count是global變量，計算逆序數(shù)時每次都會再上一次運(yùn)算的基礎(chǔ)上再加上這一次計算得到的逆序數(shù)，造成了錯誤，怎么才能改掉這個bug？謝謝！

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