LRU Cache

Descripiton:

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the
following operations: get and set.
get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise
return -1.
set(key, value) - Set or insert the value if the key is not already present. When the cache reached its
capacity, it should invalidate the least recently used item before inserting a new item.

分析：
为了使查找、插入和删除都有较高的性能,我们使用一个双向链表 (std::list) 和一个哈希表
(std::unordered_map),因为:
• 哈希表保存每个节点的地址,可以基本保证在 O(1) 时间内查找节点
• 双向链表插入和删除效率高,单向链表插入和删除时,还要查找节点的前驱节点
具体实现细节:
• 越靠近链表头部,表示节点上次访问距离现在时间最短,尾部的节点表示最近访问最少
• 访问节点时,如果节点存在,把该节点交换到链表头部,同时更新 hash 表中该节点的地址
• 插入节点时,如果 cache 的 size 达到了上限 capacity,则删除尾部节点,同时要在 hash 表中删
除对应的项;新节点插入链表头部。

#include <iostream>#include <unordered_map>#include <list>using namespace std;struct cacheNode{    int key;    int val;    cacheNode(int k,int v):key(k),val(v) {}};class LRUcache{    public:        LRUcache(int capacity)        {            this->capacity = capacity;        }        int getKey(int key);        void setKey(int key, int value);    private:        list<cacheNode> cacheList;        unordered_map<int,list<cacheNode>::iterator> cacheMap;        int capacity;};int LRUcache::getKey(int key){    if (cacheMap.find(key) == cacheMap.end())        return -1;    //move the current node to the head ,and update address in map    cacheList.splice(cacheList.begin(),cacheList,cacheMap[key]);    cacheMap[key] = cacheList.begin();    return cacheMap[key]->val;}void LRUcache::setKey(int key, int value){    if (cacheMap.find(key) == cacheMap.end())  //insert    {        if (cacheList.size() == capacity)   //delete the tail node        {            cacheMap.erase(cacheList.back().key);            cacheList.pop_back();        }        cacheList.push_front(cacheNode(key,value));        cacheMap[key] = cacheList.begin();    }    else    // update the node ,and move the node to head    {        cacheMap[key]->val = value;        cacheList.splice(cacheList.begin(),cacheList,cacheMap[key]);        cacheMap[key] = cacheList.begin();    }}int main(){    int key,val = 0;    int capacity = 5;    LRUcache lrucache(capacity);    lrucache.setKey(1,10);    lrucache.setKey(2,20);    lrucache.setKey(3,30);    lrucache.setKey(4,40);    cout<<"(1,"<<lrucache.getKey(1)<<")"<<"  ";    cout<<"(2,"<<lrucache.getKey(2)<<")"<<"  ";    cout<<"(3,"<<lrucache.getKey(3)<<")"<<"  ";    cout<<"(4,"<<lrucache.getKey(4)<<")"<<endl;    lrucache.setKey(5,50);    lrucache.setKey(6,60);    cout<<"(5,"<<lrucache.getKey(5)<<")"<<"  ";    cout<<"(6,"<<lrucache.getKey(6)<<")"<<endl;    lrucache.setKey(2,100);    cout<<"(2,"<<lrucache.getKey(2)<<")"<<endl;    cout<<"(1,"<<lrucache.getKey(1)<<")"<<"  ";     //already deleted ,return -1    return 0;}