算法--归并排序

博客：https://www.cnblogs.com/chengxiao/p/6194356.html

https://www.cnblogs.com/piperck/p/6030122.html

python实现：

def merge(left, right):    """
    params: left=[1,2,4]  right=[3,6,7]
    h=j=0: 初始化索引
    循环结束后，判断：如果左边的循环完了， 把右边剩下的循环插入结果集c  （j=3 len=3;  h=1 len=3）或者else。。。。
    """

    c = []
    h = j = 0
    while j < len(left) and h < len(right):
        if left[j] < right[h]:
            c.append(left[j])
            j += 1
        else:
            c.append(right[h])
            h += 1
    if j == len(left):
        for i in right[h:]:
            c.append(i)
    else:
        for i in left[j:]:
            c.append(i)

    return c


def merge_sort(li):
    """
    step1: 如果len=1,不用排序了，或者说已经排好了
    step2: 递归拆分
    step3: 合并
    """

    if len(li) <= 1:
        return li
    middle = len(li) // 2
    left = merge_sort(li[:middle])
    right = merge_sort(li[middle:])
    return merge(left, right)


if __name__ == '__main__':
    a = [4, 7, 8, 3, 5, 9]
    print(merge_sort(a))