兩數相加。
一開始看到題目以為是單純矩陣再印出的題目，嘗試數次發現沒這麼簡單...
可使用兩個迴圈的完成。
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].
