LRU Cache

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.

pro:

实现一个LRU Cache，要求能根据key做get操作，返回其value；能对给出的key，value如果之前的key存在，则设置value；不存在则如果Cache的容量尚有多余，直接插入，否则要淘汰最近最少用的数据，再将其插入。无论是set还是get，都视这个数据是最新被使用过。

sol:

可以预见，删除插入比较多，要做到O(1)，就需要使用链表，需要记录最后的节点（最旧的）以便删除，又需要将新节点或者最近访问的节点插入到表头后，因此需要双向链表。get的时候又需要O(1)定位，所以需要一个map，从key指向链表的节点。有了这两个数据结构就OK了~！

trick:

双向链表的删除和插入操作是基础啊~！！分别影响两条线和四条线。

code：

struct ListNodee{    int key;    int val;    ListNodee *next;    ListNodee *pre;    ListNodee(int x,int y):key(x),val(y),next(NULL),pre(NULL){}};class LRUCache{private:    int capacity;    map<int,ListNodee*> mymap;    ListNodee *root;    ListNodee *lastone;    int current;public:    LRUCache(int capacity) {        this->capacity = capacity;        this->mymap.clear();        this->root = new ListNodee(0,0);        this->lastone = root;        this->current = 0;    }        int get(int key) {        if(mymap.find(key)==mymap.end()) return -1;        else        {            ListNodee* temp = mymap[key];            makeTop(root,temp);            return temp->val;        }    }    void deletion(ListNodee *root,ListNodee *mynode)    {        //delete from map        map<int,ListNodee*>::iterator it;        it = mymap.find(mynode->key);        mymap.erase(it);                if(mynode->next==NULL) this->lastone = mynode->pre;        this->current--;        mynode->pre->next = mynode->next;        if(mynode->next!=NULL)//forgot added!!!!!!!            mynode->next->pre = mynode->pre;//forgot added!!!!!!!    }    void makeTop(ListNodee *root,ListNodee *mynode)    {        deletion(root,mynode);        insert(this->root,mynode);    }    void insert(ListNodee *root,ListNodee *mynode)    {        this->current++;        mynode->pre = root;        mynode->next = root->next;        if(root->next!=NULL)            root->next->pre = mynode;        root->next = mynode;        if(mynode->next == NULL) this->lastone = mynode;//forgot added!!!!!!!        mymap[mynode->key] = mynode;    }        void set(int key, int value) {        if(mymap.find((key))!=mymap.end())        {            ListNodee *temp = mymap[key];            temp->val = value;            makeTop(root,temp);        }else        {            ListNodee *newnode = new ListNodee(key,value);            if(this->current<this->capacity)            {                insert(this->root,newnode);            }else            {                //remove oldest,insert new,remove in map,update variables                deletion(this->root,this->lastone);                insert(this->root,newnode);            }        }    }};