思路

很容易想到的方法是二重循环遍历一遍，但是会很慢

把加法变减法可以大大加速

代码

class Solution:

    def twoSum(self, nums, target):

        """

        :type nums: List[int]

        :type target: int

        :rtype: List[int]

        """

        book = {}

        for key,value in enumerate(nums):

            if value not in book:

                delta = target - value

                book[delta] = key		#对应的key和value分别是差值和索引

            else:

                return [book[value], key]

PS.二重循环是7000+ms，上面的代码只要40ms