#切片：

# L = ['aaa', 'bbb', 'ccc', 'ddd']

# print(L[1 : 3])  #取[1, 3):下标

# L = list(range(100))

# print(L[:10])

# print(L[-10:])

#

# print(L[2:10:4])       #从2开始取，步长为4，小于10

# # [2, 6]

# print('abcdefg'[:3])

# # abc

# print('abcdefg'[::2])

# # aceg

#取出字符串的首位空格的函数

# def trim(s):

#     if s == '':

#         return ''

#     len1 = len(s)

#     i = 0

#     j = len1 - 1

#     while i < len1  and s[i] == ' ':

#         i += 1

#         print(i)

#     while j > 0 and s[j] == ' ' :

#         j -= 1

#         print(j)

#     if i <= j:

#         print(i, (j))

#         return s[i:j+1]

#     else:

#         return '' 

#递归写法：

# def trim(s):

#     if s == '':

#         return s

#     if s[:1] == ' ':

# #     if s[0] == ' ':

#         return trim(s[1:])

#     if s[-1:] == ' ':

# #     if s[-1] == ' ':

#         return trim(s[:-2])

#     else:

#         return s

#递归写法2：

def trim(s):

    if s == '':

        return s

    if s[0] == ' ':

        s = trim(s[1: ])   #如果这里不用返回值的话，必须要在下面判空，否则可能s已经为空了，再去做下一个if判断，就会越界

#         return trim(s[1: ])

    if s == '':

        return s

    if s[-1] == ' ':

        s = trim(s[ :-2])

#         return trim(s[ :-2])

    return s

#

# # 测试:

t = [1, 2, 3, 4, 5]

print(t[:-2])

if trim(' ') != '':

    print('ys' + trim('    hello') + 'ys')

    print('no')

#

if trim('hello  ') != 'hello':

    print('a' + trim('hello  ') + 'b')

    print('1测试失败!')

elif trim('  hello') != 'hello':

    print('2测试失败!')

elif trim('  hello  ') != 'hello':

    print('3测试失败!')

elif trim('  hello  world  ') != 'hello  world':

    print('4测试失败!')

elif trim('') != '':

    print('5测试失败!')

elif trim('    ') != '':

    print('6测试失败!')

else:

    print('测试成功!')