leetcode刷题（java）——day01

Two Sum

Question

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].

use java8 to solution

import java.util.Hashtable;

/*
 * @lc app=leetcode id=1 lang=java
 *
 * [1] Two Sum
 */
class Solution {
    public int[] twoSum(int[] nums, int target) {
        Hashtable<Integer,Integer> hashtable  =new Hashtable<Integer,Integer>();
        int i = 0;
        while ((i< nums.length)&& (hashtable.get(nums[i]) == null)){
            hashtable.put(target - nums[i], i);
            i++;
        }
        if(i<nums.length){
            return new int[]{hashtable.get(nums[i]),i};
        }
        return null;
    }
}

Next up is an understanding of Hash.

Hash ：散列，通过关于键值(key)的函数，将数据映射到内存存储中一个位置来访问。这个过程叫做Hash，这个映射函数称做散列函数，存放记录的数组称做散列表(Hash Table),又叫哈希表。

hash相对于数组和链表最大的好处就是，先对数据进行分类，再进行查找，减少了遍历的过程，从而大大减少了寻值的时间。