LeetCode:LRU Cache

Posted 2020-07-29 walker lee

LRU Cache

Total Accepted: 76226 Total Submissions: 481333 Difficulty: Hard

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.

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 Design

思路：

题目要求实现“最近最少使用”缓存算法，这在android中的图片缓存中有使用。

实现要求是，最近被访问（get,set）的放前后；最久被访问的放在最后，但容量不足时删除此结点。

实现方式：双链表 + HashMap

java code:

class Node {
    int key,value;
    Node pre,next;
    
    public Node(int key, int value) {
        this.key = key;
        this.value = value;
    }
}

public class LRUCache {
    
    HashMap<Integer, Node> map;
    int capicity;
    int count;
    Node head,tail;
    
    // 设置一个头结点和一个尾结点，作为哨兵
    public LRUCache(int capacity) {
        this.capicity = capacity;
        map = new HashMap<>();
        head = new Node(0, 0);
        tail = new Node(0, 0);
        
        head.next = tail;
        tail.pre = head;
        head.pre = null;
        tail.next = null;
        count = 0;
    }
    
    // 删除结点
    public void deleteNode(Node node) {
        node.pre.next = node.next;
        node.next.pre = node.pre;
    }
    
    // 添加，添加到head的下一个结点
    public void addToHead(Node node) {
        node.pre = head;
        node.next = head.next;
    
        head.next.pre = node;
        head.next = node;
    }
    
    // 
    public int get(int key) {
        if(map.get(key)!=null) {
            Node node = map.get(key);
            int result = node.value;
            deleteNode(node);
            addToHead(node);
            return result;
        }
        return -1;
    }
    
    // 
    public void set(int key, int value) {
        if(map.get(key)!=null) {
            Node node = map.get(key);
            node.value = value;
            deleteNode(node);
            addToHead(node);
        }else{
            Node node = new Node(key, value);
            map.put(key, node);
            if(count < capicity) {
                count++;
                addToHead(node);
            }else{
                map.remove(tail.pre.key);
                deleteNode(tail.pre);
                addToHead(node);
            }
        }
    }
}

集合类中LinkedHashMap的实现方式就是：双链表 + HashMap，也是Android LRU Cache中实现使用数据结构，因此只需在LinkedHashMap的基础上添加容量限制即可。

java code:

public class LRUCache {

    private Map<Integer, Integer> map;

    public LRUCache(int capacity) {
        map = new LinkedCappedHashMap<>(capacity);
    }

    public int get(int key) {
        if(!map.containsKey(key)) { return -1; }
        return map.get(key);
    }

    public void set(int key, int value) {
        map.put(key,value);
    }

    private static class LinkedCappedHashMap<K,V> extends LinkedHashMap<K,V> {

        int maximumCapacity;

        LinkedCappedHashMap(int maximumCapacity) {
            super(16, 0.75f, true);
            this.maximumCapacity = maximumCapacity;
        }

        protected boolean removeEldestEntry(Map.Entry eldest) {
            return size() > maximumCapacity;
        }
    }
}