#Created By: Chen Da

#定義一個合併方法，等價於Python內建的sorted
def merge_sorted_list(sorted_a,sorted_b):
    length_a,length_b = len(sorted_a),len(sorted_b)
    a = b = 0                               #定義指標
    new_list = list()

    while a < length_a and b < length_b:
        if sorted_a[a] < sorted_b[b]:
            new_list.append(sorted_a[a])
            a += 1
        else:
            new_list.append(sorted_b[b])
            b += 1

    while a < length_a:
        new_list.append(sorted_a[a])
        a += 1

    while b < length_b:
        new_list.append(sorted_b[b])
        b += 1
    return new_list

#合併排序
def merge_sort(seq):
    if len(seq) <= 1:
        return seq
    else:
        mid = int(len(seq) / 2)
        left_sorted_list = merge_sort(seq[:mid])            #遞迴等分原列表
        right_sorted_list = merge_sort(seq[mid:])
        new_sorted_list = merge_sorted_list(left_sorted_list,right_sorted_list)
        return new_sorted_list


def test_merge_sort():
    lyst = list(range(20))
    import random
    random.shuffle(lyst)

    assert merge_sort(lyst) == list(range(20))

if __name__ == "__main__":
    test_merge_sort()