1. Two Sum

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