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

'''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))

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

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