146.LRU缓存

题目：

请你设计并实现一个满足  LRU (最近最少使用) 缓存 约束的数据结构。

实现 LRUCache 类：

• LRUCache(int capacity) 以 正整数 作为容量 capacity 初始化 LRU 缓存
• int get(int key) 如果关键字 key 存在于缓存中，则返回关键字的值，否则返回 -1 。
• void put(int key, int value) 如果关键字 key 已经存在，则变更其数据值 value ；如果不存在，则向缓存中插入该组 key-value 。如果插入操作导致关键字数量超过 capacity ，则应该 逐出 最久未使用的关键字。

函数 get 和 put 必须以 O(1) 的平均时间复杂度运行。

示例：

输入
["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
输出
[null, null, null, 1, null, -1, null, -1, 3, 4]

解释
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // 缓存是 {1=1}
lRUCache.put(2, 2); // 缓存是 {1=1, 2=2}
lRUCache.get(1);    // 返回 1
lRUCache.put(3, 3); // 该操作会使得关键字 2 作废，缓存是 {1=1, 3=3}
lRUCache.get(2);    // 返回 -1 (未找到)
lRUCache.put(4, 4); // 该操作会使得关键字 1 作废，缓存是 {4=4, 3=3}
lRUCache.get(1);    // 返回 -1 (未找到)
lRUCache.get(3);    // 返回 3
lRUCache.get(4);    // 返回 4

LCR 031. LRU 缓存 - 力扣（LeetCode）

题解：

思路：

用一个哈希表和一个双端队列，队列里面存储key，哈希存value。get时把对应的key删除，放在队首。put时有旧的则删掉，size够了把队列的最后一个删掉。然后加入，并且覆盖哈希表里面的值。

代码：

class LRUCache {
 
    private int capacity;
    private Map<Integer,Integer> cache;
    private Deque<Integer> queue;
    
    public LRUCache(int capacity) {
        this.capacity=capacity;
        this.cache=new HashMap<>();
        this.queue=new LinkedList<>();
    }
    
    public int get(int key) {
        if(cache.containsKey(key)){
            queue.remove(key);
            queue.offerFirst(key);
            return cache.get(key);
        }
        return -1;
    }
    
    public void put(int y, int value) {
        if(cache.containsKey(y)){
            queue.remove(y);
        }
        if(queue.size()==capacity){
            Integer oldKey=queue.pollLast();
            cache.remove(oldKey);
        }
        cache.put(y,value);
        queue.offerFirst(y);
    }
}

class Node {
    int data;
    Node next;
    Node prev;
    
    Node(int data) {
        this.data = data;
    }
}
 
class MyDeque {
    private Node front;
    private Node rear;
    private int size;
    
    public MyDeque() {
        front = null;
        rear = null;
        size = 0;
    }
    
    // 向队列前端添加元素
    public void offerFirst(int data) {
        Node newNode = new Node(data);
        if (isEmpty()) {
            front = newNode;
            rear = newNode;
        } else {
            newNode.next = front;
            front.prev = newNode;
            front = newNode;
        }
        size++;
    }
    
    // 移除指定元素
    public void remove(int key) {
        Node current = front;
        while (current != null) {
            if (current.data == key) {
                if (current == front) {
                    pollFirst();
                } else if (current == rear) {
                    pollLast();
                } else {
                    current.prev.next = current.next;
                    current.next.prev = current.prev;
                    size--;
                }
                return;
            }
            current = current.next;
        }
        return;
    }
    
 // 从队列尾部移除元素
    public int pollLast() {
        if (isEmpty()) {
            return -1;
        }
        int removedData = rear.data;
        if (size == 1) {
            front = null;
            rear = null;
        } else {
            rear = rear.prev;
            rear.next = null;
        }
        size--;
        return removedData;
    }
 // 从队列前端移除元素
    public int pollFirst() {
        if (isEmpty()) {
            return -1;
        }
        int removedData = front.data;
        if (size == 1) {
            front = null;
            rear = null;
        } else {
            front = front.next;
            front.prev = null;
        }
        size--;
        return removedData;
    }
    // 判断队列是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    
    // 获取队列大小
    public int size() {
        return size;
    }
    
}

完整的双端队列：

class Node {
    int data;
    Node next;
    Node prev;
    
    Node(int data) {
        this.data = data;
    }
}
 
public class MyDeque {
    private Node front;
    private Node rear;
    private int size;
    
    public MyDeque() {
        front = null;
        rear = null;
        size = 0;
    }
    
    // 向队列前端添加元素
    public void addFirst(int data) {
        Node newNode = new Node(data);
        if (isEmpty()) {
            front = newNode;
            rear = newNode;
        } else {
            newNode.next = front;
            front.prev = newNode;
            front = newNode;
        }
        size++;
    }
    
    // 向队列尾部添加元素
    public void addLast(int data) {
        Node newNode = new Node(data);
        if (isEmpty()) {
            front = newNode;
            rear = newNode;
        } else {
            rear.next = newNode;
            newNode.prev = rear;
            rear = newNode;
        }
        size++;
    }
    
    // 从队列前端移除元素
    public int removeFirst() {
        if (isEmpty()) {
            throw new IllegalStateException("队列为空");
        }
        int removedData = front.data;
        if (size == 1) {
            front = null;
            rear = null;
        } else {
            front = front.next;
            front.prev = null;
        }
        size--;
        return removedData;
    }
    
    // 从队列尾部移除元素
    public int removeLast() {
        if (isEmpty()) {
            throw new IllegalStateException("队列为空");
        }
        int removedData = rear.data;
        if (size == 1) {
            front = null;
            rear = null;
        } else {
            rear = rear.prev;
            rear.next = null;
        }
        size--;
        return removedData;
    }
    
    // 移除指定元素
    public void remove(int key) {
        Node current = front;
        while (current != null) {
            if (current.data == key) {
                if (current == front) {
                    removeFirst();
                } else if (current == rear) {
                    removeLast();
                } else {
                    current.prev.next = current.next;
                    current.next.prev = current.prev;
                    size--;
                }
                return;
            }
            current = current.next;
        }
        throw new IllegalArgumentException("队列中没有该元素");
    }
    
    // 获取队列前端元素
    public int getFirst() {
        if (isEmpty()) {
            throw new IllegalStateException("队列为空");
        }
        return front.data;
    }
    
    // 获取队列尾部元素
    public int getLast() {
        if (isEmpty()) {
            throw new IllegalStateException("队列为空");
        }
        return rear.data;
    }
    
    // 判断队列是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    
    // 获取队列大小
    public int size() {
        return size;
    }
    
}