计算一个tuple里面的逆序数，用merge sort的办法。我写了以下代码，但是每次统计的时候，count设置为全局变量了：

'''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))

如果只运行一次，那么这个程序是对的。但是如果要运行好几次，由于count是global变量，计算逆序数时每次都会再上一次运算的基础上再加上这一次计算得到的逆序数，造成了错误，怎么才能改掉这个bug？谢谢！