剑指Offer-51 数组中的逆序对

题目：

在数组中的两个数字如果前面一个数字大于后面的数字，则这两个数字组成一个逆序对。输入一个数组，求出这个数组中的逆序对的总数。
样例
输入：[1,2,3,4,5,6,0]
输出：6

解答：

class Solution(object):
    def inversePairs(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums) <= 1:
            return 0
        count = 0
        mid = int(len(nums)/2)
        self.copy = nums[:]
        count = self.inversePairsCore(nums, 0, len(nums) - 1)
        return count
    
    def inversePairsCore(self, seq, start, end):
        if start == end:
            return 0
        mid = int((start + end)/2)
        count = 0
        count += self.inversePairsCore(seq, start, mid) + self.inversePairsCore(seq, mid + 1, end)
        i, j = mid, end
        index = end
        while(i >= start and j >= mid + 1):
            if seq[i] > seq[j]:
                self.copy[index] = seq[i]
                count += j - mid
                i -= 1
            else:
                self.copy[index] = seq[j]
                j -= 1
            index -= 1
        while(i >= start):
            self.copy[index] = seq[i]
            index -= 1
            i -= 1
        while(j >= mid + 1):
            self.copy[index] = seq[j]
            index -= 1
            j -= 1
        for i in range(start, end + 1):
            seq[i] = self.copy[i]
        return count