計算逆序對的個數

!/usr/bin/python

'''file name: inversion.py
   --P41
   --calculate the inversion number in nlgn for the worse situation
   --author: zevolo, 2012.05.07
'''
global count

def merge_sort(list, s, e):
    global count
    if e == s:
        return
    m = (e + s) / 2
    #print "sort %d-->%d   call %d, %d   %d, %d" % (s, e, s, m, m+1, e)
    merge_sort(list, s, m)
    merge_sort(list, m + 1, e)

    l = list[s : m+1]
    r = list[m+1 : e+1]

    p1 = 0
    p2 = 0
    for i in range(s, e+1):
        if p1 >= (m + 1 - s):
            list[i] = r[p2]
            p2 += 1
        elif p2 >= (e - m):
            list[i] = l[p1]
            p1 += 1
            count += 1
        elif l[p1] <= r[p2]:
            list[i] = l[p1]
            p1 += 1
        else:
            list[i] = r[p2]
            p2 += 1
            count += 1

if __name__ == '__main__':
    global count
    count = 0
    l = [0, 3, 1, 9,7,5,2,8, 10,6,4,11]
    merge_sort(l, 0, len(l) - 1)
    print l
    print "count is %d" % (count,)