indicesof the two numbers such that they add up to a specific target.
You may assume that each input would haveexactlyone solution, and you may not use thesameelement twice.
Example:
Given nums = [2, 7, 11, 15], target = 9,Because nums[0] + nums[1] = 2 + 7 = 9,return [0,1].
O(n*n)时间复杂度
