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.

Code

import java.util.*;
public class LRUCache {
    class DLinkedNode{
        int key;
        int value;
        DLinkedNode pre;
        DLinkedNode post;
    }

    //always add the new node right after head
    private void addNode(DLinkedNode node){
        node.pre = head;
        node.post = head.post;
        head.post.pre = node;
        head.post = node;
    }

    //remove existing node from the linked list
    private void removeNode(DLinkedNode node){
        DLinkedNode pre = node.pre;
        DLinkedNode post = node.post;
        pre.post = post;
        post.pre = pre;
    }

    //move certain node in between to the head
    private void moveToHead(DLinkedNode node){
        this.removeNode(node);
        this.addNode(node);
    }

    //pop the current tail
    private DLinkedNode popTail(){
        DLinkedNode res = tail.pre;
        this.removeNode(res);
        return res;
    }

    private Hashtable<Integer, DLinkedNode> cache = new Hashtable<Integer, DLinkedNode>();
    private int count;
    private int capacity;
    private DLinkedNode head, tail;

    public LRUCache(int capacity) {
        this.count = 0;
        this.capacity = capacity;
        head = new DLinkedNode();
        head.pre = null;
        tail = new DLinkedNode();
        tail.post = null;
        head.post = tail;
        tail.pre = head;
    }

    public int get(int key) {
        DLinkedNode node = cache.get(key);
        if(node==null) return -1;
        this.moveToHead(node);
        return node.value;
    }

    public void set(int key, int value) {
        DLinkedNode node = cache.get(key);
        if(node==null){
            DLinkedNode newNode = new DLinkedNode();
            newNode.key = key;
            newNode.value = value;

            this.cache.put(key, newNode);
            this.addNode(newNode);

            ++count;

            if (count>capacity){
                DLinkedNode tail = this.popTail();
                this.cache.remove(tail.key);
                --count;
            }
        }else {
            node.value = value;
            this.moveToHead(node);
        }
    }
}

Reference