class Solution(object):

    def twoSum(self, nums, target):

        """

        :type nums: List[int] nums=[2,7,11,15],target=9

        :type target: int

        :rtype: List[int]

        """

        hashmap={}

        for index,num in enumerate(nums):

            another_num=target-num

            if another_num in hashmap:

                return [hashmap[another_num],index]

            hashmap[num]=index

        return None

笔记：

1、enumerate用法：这里其实不知道他这个循环nummerate（nums）到底能不能取出来key和value

>>> a=[2,3]
>>> b=enumerate(a)
>>> b
<enumerate object at 0x00000000031C6750>
>>> list(b)
[(0, 2), (1, 3)]
>>>

2、思想是在[2,7,11,15]里取出来第一个值，放在hashmap里，用target9减去第二个值，如果在hashmap里这个差值的存在说明满足2+7=9，然后把这个hashmap里的key-value输出来。

class Solution(object):

    def reverse(self, x):

        """

        :type x: int

        :rtype: int

        """

        if x<0:

            tmp=-int(str(-x)[::-1])

        else :

            tmp=int(str(x)[::-1])

        if tmp>=-2**31 and tmp<=2**31-1:

            return tmp

        else:

            return 0

>>> x

-123

>>> str(x)

'-123'

>>> b=str(x)

>>> b[::-1]

'321-'

>>> x

-123

>>> x=-x

>>> x

123

>>> str(x)

''

>>> b=str(x)

>>> b

''

>>> b[::-1]

''

>>>