def list2str(lst: list) -> str:
    """
    将 list 内的元素转化为字符串，使得打印时能够按行输出并在前面加上序号(从1开始)

    e.g.
    In:
    lst = [a,b,c]
    str = list2str(lst)
    print(str)

    Out:
    1. a
    2. b
    3. c
    """
    i = 1
    res_list = []
    for x in lst:
        res_list.append(str(i)+'. '+str(x))
        i += 1
    res_str = '\n'.join(res_list)
    return res_str