[LeetCode]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.

思考：第一次遇到这种题目，说实话我实在构造不出这么好的数据结构：map+双向链表。

struct Node{
    int key;
    int value;
    Node *pre;
    Node *next;
    Node(int a,int b):key(a),value(b),pre(NULL),next(NULL){};
};//双向链表数据结构

class LRUCache{
private:
    int maxsize;
    Node *head;//链表头结点
    Node *tail;//链表尾结点
    map<int,Node*> m;
    
public:
    LRUCache(int capacity) {
        maxsize=capacity;
        head=NULL;
        tail=NULL;
        m.clear();
    }
    
    int get(int key) {
        map<int,Node*>::iterator iter=m.find(key);
        if(iter!=m.end())//找到key返回value，将结点移到链表头部
        {
            move(iter->second);
            return iter->second->value;
        }
        else return -1;
    }
    
    void set(int key, int value) {
        map<int,Node*>::iterator iter=m.find(key);
        if(iter!=m.end())//已存在key
        {
            Node *p=iter->second;
            p->value=value;
            move(p);
        }
        else//不存在key
        {
            if(m.size()==maxsize)//链表满删除尾结点
            {
                iter=m.find(tail->key);
                m.erase(iter);
                Node *p=tail;
                if(p->pre==NULL) head=NULL; 
                else p->pre->next=NULL;
				tail=tail->pre;
                delete p;
            }
            insert(key,value);
        }
    }
    
    void insert(int key,int value)//插入到头结点
    {
        Node *node=new Node(key,value);
        if(head==NULL) head=tail=node;
        else
        {
			head->pre=node;
            node->pre=NULL;
            node->next=head;            
            head=node;
        }
        m.insert(make_pair(key,node));
    }
    
    void move(Node *node)//移动到头结点
    {
        if(node==head) return;
        if(node==tail) tail=node->pre;
        node->pre->next=node->next;
        if(node->next!=NULL) node->next->pre=node->pre;
        node->pre=NULL;
        node->next=head;
        head->pre=node;
        head=node;
    }
};