归并排序求逆序对python

归并排序求逆序对的Python代码如下：

def merge_sort(arr):
    if len(arr) <= 1:
        return arr, 0
    mid = len(arr) // 2
    left, left_count = merge_sort(arr[:mid])
    right, right_count = merge_sort(arr[mid:])
    merged, merge_count = merge(left, right)
    return merged, left_count + right_count + merge_count

def merge(left, right):
    result = []
    i, j, count = 0, 0, 0
    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1
            count += len(left) - i
    result += left[i:]
    result += right[j:]
    return result, count

其中，`merge_sort` 函数返回排好序的数组和逆序对的数量，`merge` 函数用于合并两个有序数组并计算逆序对的数量。