Two sum

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:

演练

1. 先审题
输入：系列的数组，和一个target的结果
输出： 从数组中能拿到两个数字之和等于这个target

2. 第一反应