104-LRU Cache

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–146. LRU Cache My Submissions QuestionEditorial Solution
Total Accepted: 74197 Total Submissions: 469395 Difficulty: Hard
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.

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设计一个key-value类型的LRU缓存：
1.首先由key-value要求可定义结构体节点
2.如何组织这些节点便于
a.快速插入或者替换
b.快速随机读取某个节点，比如从key读取value
如果cache不满：插入
如果cache满了：替换最老的
同时保持最近访问的在前面
从操作来看，要插入和随机读取并行
这个，自然想到链表，又要随机读取，那就只有靠hash了
3.如何设计数据结构
如下详解：

class LRUCache{private:    struct CacheNode{//设定key-value节点      int key;      int value;      CacheNode(int _k,int _v):key(_k),value(_v){};    };public:    LRUCache(int capacity) {//设定cache的容量        this->capacity=capacity;    }    int get(int key) {//随机访问如何做到？通过key做hash对象利用hash找到节点地址，再访问value        if(CacheMap.find(key)==CacheMap.end())//如果不在里面返回-1表示无            return -1;        else {//如果在呢，自然是重新排序缓存列表，然后重建索引            CacheList.splice(CacheList.begin(),CacheList,CacheMap[key]);            CacheMap[key]=CacheList.begin();            return CacheMap[key]->value;        }    }    void set(int key, int value) {        if(CacheMap.find(key)==CacheMap.end()){//如果不在cache里，自然是插入到cache            if(CacheList.size()==capacity){//cache满了呢？替换最老的                CacheMap.erase(CacheList.back().key);                CacheList.pop_back();            }            CacheList.push_front(CacheNode(key,value));//在头部插入新的，因为刚访问的优先级高            CacheMap[key]=CacheList.begin();        }        else{     //本来就有key-value的映射又set，所以就是更新相应的值，并加到列表首部，刚访问的嘛            CacheMap[key]->value=value;            CacheList.splice(CacheList.begin(),CacheList,CacheMap[key]);            CacheMap[key]=CacheList.begin();        }    }    list<CacheNode> CacheList;    unordered_map<int,list<CacheNode>::iterator> CacheMap;    int capacity;};

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