1. Two Sum.md

1. Two Sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.
给一个整数数组，返回其中两个数的索引，这两个数的和为给定的target。

You may assume that each input would have exactly one solution, and you may not use the same element twice.
你可以认为给定的输入只会有一组解，并且你不能用同一个数两次

Example:

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

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

Solution