1. Two Sum (map )

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

20180325
用 key in dict: 判断效率更高！！！
时间复杂度 o(n)
空间复杂度 o(n)

class Solution:
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        n_dict = {}
        for i,item in enumerate(nums):
            val = target - item
            if item in n_dict:
                return [n_dict[item],i]
            else:
                n_dict[val] = i