python-实现归并排序

# encoding=utf-8


def merge_sort(alist):
    """归并排序"""
    n = len(alist)

    if n == 1:
        return alist
    mid = n // 2

    # 对分割的左半部分进行归并排序
    left_sorted_li = merge_sort(alist[:mid])
    # 对分割的右半部分进行归并排序
    right_sorted_li = merge_sort(alist[mid:])

    # 对排序之后的两部分进行合并
    # 定义两个游标
    left, right = 0, 0

    merge_result_li = []
    left_n = len(left_sorted_li)
    right_n = len(right_sorted_li)

    while left < left_n and right < right_n:
        if left_sorted_li[left] <= right_sorted_li[right]:
            merge_result_li.append(left_sorted_li[left])
            left += 1
        else:
            merge_result_li.append(right_sorted_li[right])
            right += 1

    merge_result_li += left_sorted_li[left:]
    merge_result_li += right_sorted_li[right:]

    # 将合并后的结果　一个新的类别返回
    return merge_result_li


if __name__ == '__main__':
    alist = [22, 1, 4, 553, 3, 212, 77]
    print("before alist: %s" % alist)
    sorted_alist = merge_sort(alist)
    print("after alist: %s" % alist)
    print("sorted new list %s" % sorted_alist)