LeetCode 146. LRU cache

Topic link ：https://leetcode-cn.com/problems/lru-cache/

cache （Cache） It is a technology to improve data reading performance , In hardware design 、 It is widely used in software development , For example, the common CPU cache 、 Database cache 、 Browser caching and so on .

The size of the cache is limited . When the cache is full , What data should be cleaned out , What data should be kept , This requires a cache obsolescence strategy .

FIFO（First In First Out）, First in, first out strategy

thought ： If a data is the first to enter , So it can be thought that in the future it is very unlikely to be visited . When the space is full , The data that enters first will be eliminated first .

LFU（Least Frequently Used）, Least recently used strategy

thought ： If data is rarely accessed in a recent period of time , So it can be thought that it is unlikely to be visited in the future . therefore , When the space is full , The least frequently accessed data is the first to be eliminated .

LRU（Least Recently Used）, The last time you haven't used a strategy recently

thought ： If a data has not been accessed in a recent period of time , So it can be thought that it is unlikely to be visited in the future . therefore , When the space is full , The data that has not been accessed for the longest time is eliminated first .

Intuitive ideas ： Store the most recently used elements near the top of the cache , When a new entry is accessed ,LRU Put it at the top of the cache . When the cache is full , Entries accessed earlier will be removed from the bottom of the cache .

Hashtable + Array

Hashtable + Single chain list

Hashtable + Double linked list