思考练习

Question：
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].

Solution：
1：双重循环，获取所有元素，两两组合，判断运算是否成立，获取下标值（笨办法，效率低）
2：利用HashMap