字符串可使用sorted()函数

s = "huhongiang"

sorted(s)

['a', 'g', 'g', 'h', 'h', 'i', 'n', 'n', 'o', 'u']

l = sorted(s)

l

['a', 'g', 'g', 'h', 'h', 'i', 'n', 'n', 'o', 'u']

习题1："abcdefgh"里面挑出3个字母进行组合，一共有多少种组合，要求三个字母中不能有任何重复的字母，三个字母的同时出现次数，在所有组合中只能出现一次，例如出现abc了，不能出现cab和bca等。

方式1：

s = "abcdefgh"result = []temp_list = []for k in s:    for v in s:        for j in s:            if k != v and k != j and v !=j:                #sorted("abc") ==》["a","b","c"]                #[sorted(s) for s in result]每一个元素是一个包含三个字母的列表                #if sorted(list(k+v+j))  not in [sorted(list(s)) for s in result]                if sorted(list(k+v+j)) not in [sorted(s) for s in result]:                    result.append(k+v+j)                    #print(result)print("组合数:",len(result))

方式2：

s = "abcdefgh"result = []temp_list = []for k in s:    for v in s:        for j in s:            if k != v and k != j and v !=j:                #join后直接比较字符串                if "".join(sorted(list(k+v+j)))  not in ["".join(sorted(list(s))) for s in result]:                    result.append(k+v+j)                    #print(result)print("组合数:",len(result))

习题2：复制一个列表，不能使用切片和复制的函数进行赋值，尽可能使用少的内置函数

方式1:

lst = ["a","b","c","d","e"]length = 0i = 0for v in lst:    length += 1result = [None]*lengthwhile i< length:    result[i] = lst[i]    i += 1print("复制后的列表为:",result)

方式2：

a = ["a","b","c","d","e"]arr_length=0for i in a:    arr_length+=1def iter_arr(n):    arr = []    i = 0    while i<=n-1:        arr+=[i]        i+=1    return arrresult = [""]*arr_lengthfor i in iter_arr(arr_length):    result[i] = a[i]print(result)

枚举：enumerate

可以遍历元素对应的行号 和元素lst_1 = [1,2,3,["a","b"]]for idx,v in enumerate(lst_1):print(idx,v)迭代器

可迭代列表、元组、字符串及字典，字典只迭代key

可使用next() next()进行输出元素

使用next()输出元素

列表

l = [5,6,7]

it = iter(l)

it.next()

5

it.next()

6

it.next()

7

没有元素时候迭代器会报错

it.next()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

StopIteration

字典:

it = iter({1:2,3:4})

it.next()

1

it.next()

3

it.next()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

StopIteration

it =iter({1:2,3:4}.values())

it.next()

2

it.next()

4

it.next()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

StopIteration

使用next()输出元素

l = [5,6,7]

it = iter(l)

next(it)

5

next(it)

6

next(it)

7

next(it)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

StopIteration

遍历迭代器

l = [1,2,3,4,6]

it = iter(l)

for i in it:

... print(i)

...

1

2

3

4

6

自定义迭代器

iter()和next()方法

这两个方法是迭代器最基本的方法：

➢ 一个用来获得迭代器对象

➢ 一个用来获取容器中的下一个元素

#encoding=utf-8class MyIter(object):    def __init__(self,n):        self.idx = 0        self.n = n    def __iter__(self):        return self    def __next__(self):        if self.idx