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, 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].

My answer:

class Solution {
    public int[] twoSum(int[] nums, int target) {
       for (int i = 0; i < nums.length; i++)
            for (int j = i+1; j < nums.length; j++) {
                if (nums[i] == target - nums[j])
                    return new int[]{i, j};

                continue;
            }
        return null;
    }
}

Others: 使用hashmap,遍历数组,将target-arr[i]插入hashmap,判断下一个元素是否在hashmap中