Two Sum

方法1：

时间复杂度： O(n)

空间复杂度：O(n)

class Solution:
    # time: O(n), space O(n)
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        dict = {}
        for i in range(len(nums)):
            if target - nums[i] in dict:
                return [dict[target - nums[i]], i]
            dict[nums[i]] = i