力扣 146. LRU缓存机制 双向链表+哈希

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

思路：使用双向链表模拟队列，当执行获取或者写入操作时，将对应的节点放到头节点之后(队头)。为了做到$O(1)$获取数据、$O(1)$写入数据，我们还需要一个哈希表，建立从$key$到双向链表节点的映射。

class LRUCache {
public:
    LRUCache(int capacity):c(capacity) {

    }
    
    int get(int key) {
        if(m.find(key)==m.end())
            return -1;
        moveToHead(key);
        return m[key]->second;
    }
    
    void put(int key, int value) {
        if(m.find(key)==m.end()){
            l.push_front(pr(key,value));
            m[key]=l.begin();
            if(l.size()>c){
                m.erase(l.back().first);
                l.pop_back();
            }
        }
        else{
            m[key]->second=value;
            moveToHead(key);
        }
    }

    void moveToHead(int key){
        l.push_front(*m[key]);
        l.erase(m[key]);
        m[key]=l.begin();
    }
private:
    using pr=pair<int,int>;
    using it=list<pr>::iterator;
    //双向链表
    list<pr> l;
    //哈希表
    unordered_map<int,it> m;
    //容量
    int c;
};

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache* obj = new LRUCache(capacity);
 * int param_1 = obj->get(key);
 * obj->put(key,value);
 */

或者自己实现一个双向链表：

class LRUCache {
private:
    struct node{
        int key,value;
        node *nxt,*pre;
        node():key(0),value(0),nxt(nullptr),pre(nullptr){}
        node(int key,int value):key(key),value(value),nxt(nullptr),pre(nullptr){}
    };
    //伪头尾节点
    node head,tail;
    unordered_map<int,node*> m;
    //容量和当前节点个数
    int c,siz;

public:
    LRUCache(int capacity):c(capacity),siz(0) {
        head.nxt=&tail;
        tail.pre=&head;
    }
    
    int get(int key) {
        if(m.find(key)==m.end())
            return -1;
        node *tmp=m[key];
        moveToHead(tmp);
        return tmp->value;
    }
    
    void put(int key, int value) {
        if(m.find(key)==m.end()){
            node *tmp=new node(key,value);
            addHead(tmp);
            m[key]=tmp;
            if(++siz>c){
                removeTail();
                --siz;
            }
        }
        else{
            node *tmp=m[key];
            tmp->value=value;
            moveToHead(tmp);
        }
    }

    void removeNode(node* n){
        n->pre->nxt=n->nxt;
        n->nxt->pre=n->pre;
    }

    void addHead(node* n){
        n->nxt=head.nxt;
        head.nxt->pre=n;
        head.nxt=n;
        n->pre=&head;
    }

    void moveToHead(node* n){
        removeNode(n);
        addHead(n);
    }

    void removeTail(){
        node *tmp=tail.pre;
        removeNode(tmp);
        m.erase(tmp->key);
        delete tmp;
    }
};

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache* obj = new LRUCache(capacity);
 * int param_1 = obj->get(key);
 * obj->put(key,value);
 */