1. Hash+双向链表

1.1 Java实现:

public class LRUCache {
    class DLinkedNode {
        int key;
        int value;
        DLinkedNode pre;
        DLinkedNode next;
        DLinkedNode() {
            this.key = 0;
            this.value = 0;
            pre = null;
            next = null;
        }
        DLinkedNode(int key, int value) {
            this.key = key;
            this.value = value;
            pre = null;
            next = null;
        }
    }
    private ConcurrentMap<Integer, DLinkedNode> cache = new ConcurrentHashMap<Integer, DLinkedNode>();
    private int size;
    private int capacity;
    private DLinkedNode head, tail;
    public LRUCache(int capacity) {
        this.size = 0;
        this.capacity = capacity;
        head = new DLinkedNode();
        head.pre = null;
        tail = new DLinkedNode();
        tail.next = null;
        head.next = tail;
        tail.pre = head;
    }
    public int get(String key) {
        DLinkedNode node = cache.get(key);
        if (node == null) {
            return -1; // should raise exception here.
        }
        moveToHead(node);
        return node.value;
    }
    public void put(int key, int value) {
        if (cache.containsKey(key)) {
            DLinkedNode node = cache.get(key);
            node.value = value;
            moveToHead(node);
        } else {
            DLinkedNode node = new DLinkedNode(key, value);
            cache.put(key, node);
            addToHead(node);
            ++size;
            if (size > capacity) {
                DLinkedNode tail = removeTail();
                cache.remove(tail.key);
                size--;
            }
        }
    }
    private void moveToHead(DLinkedNode node) {
        removeNode(node);
        addToHead(node);
    }
    private void addToHead(DLinkedNode node) {
        node.pre = head;
        node.next = head.next;
        head.next.pre = node;
        head.next = node;
    }
    private void removeNode(DLinkedNode node) {
        DLinkedNode pre = node.pre;
        DLinkedNode next = node.next;
        pre.next = next;
        next.pre = pre;
    }
    private DLinkedNode removeTail() {
        DLinkedNode node = tail.pre;
        removeNode(node);
        return node;
    }
}

1.2. C++实现

struct DLinkedListNode {
    DLinkedListNode *next, *prev;
    int key, val;
    DLinkedListNode() : key(0), val(0), next(nullptr), prev(nullptr) {}
    DLinkedListNode(int key, int val) : key(key), val(val), next(nullptr), prev(nullptr) {}
};
class LRUCache {
private:
    unordered_map<int, DLinkedListNode *> cache;
    DLinkedListNode *head, *tail;
    int size, capacity;
public:
    LRUCache(int capacity) : capacity(capacity), size(0) {
        head = new DLinkedListNode();
        tail = new DLinkedListNode();
        head->next = tail;
        tail->prev = head;
    }
    int get(int key) {
        if (!cache.count(key)) {
            return -1;
        }
        DLinkedListNode *node = cache[key];
        moveToHead(node);
        return node->val;
    }
    void put(int key, int value) {
        if (!cache.count(key)) {
            DLinkedListNode *node = new DLinkedListNode(key, value);
            cache[key] = node;
            ++size;
            addToHead(node);
            if (size > capacity) {
                DLinkedListNode *node = removeTail();
                cache.erase(node->key);
                delete node;
                size -= 1;
            }
        } else {
            DLinkedListNode *node = cache[key];
            node->val = value;
            moveToHead(node);
        }
    }
    void moveToHead(DLinkedListNode *node) {
        removeNode(node);
        addToHead(node);
    }
    void removeNode(DLinkedListNode *node) {
//1-->2-->3
        node->prev->next = node->next;
        node->next->prev = node->prev;
    }
    void addToHead(DLinkedListNode *node) {
        node->prev = head;
        node->next = head->next;
//        head->next->prev = node;
        node->next->prev = node;// try one time;
        head->next = node;
    }
    DLinkedListNode *removeTail() {
        DLinkedListNode *node = tail->prev;
        removeNode(node);
        return node;
    }
};