Leetcode

1. Two Sum

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.

Example:

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

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

solution:

1. 遍历（O(n2)）
2. hashmap(O(n)) 边检索边建map，遍历一遍