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.

The LRU cache is a hash table of keys and double linked nodes. The hash table 

makes the time of get() to be O(1). The list of double linked nodes make the nodes 

adding/removal operations O(1).

public class LRUCache {
    class Node{
        int key;
        int value;
        Node pre;
        Node next;
        public Node(int key, int value){
            this.key = key;
            this.value = value;
        }
    }
    int capacity;
    HashMap<Integer, Node> map = new HashMap<Integer, Node>();
    Node head=null;
    Node end=null;
 
    public LRUCache(int capacity) {
        this.capacity = capacity;
    }
    public int get(int key) {
        if(map.containsKey(key)){
            Node n = map.get(key);
            remove(n);
            setHead(n);
            return n.value;
        }
        return -1;
    }
    public void remove(Node n){
        if(n.pre!=null)
            n.pre.next = n.next;
        else
            head = n.next;
        if(n.next!=null)
            n.next.pre = n.pre;
        else
            end = n.pre;
    }
    public void setHead(Node n){
        n.next = head;
        n.pre = null;
        if(head!=null)
            head.pre = n;
        head = n;
        if(end ==null)
            end = head;
    }
    public void set(int key, int value) {
        if(map.containsKey(key)){
            Node old = map.get(key);
            old.value = value;
            remove(old);
            setHead(old);
        }else{
            Node created = new Node(key, value);
            if(map.size()>=capacity){
                map.remove(end.key);
                remove(end);
                setHead(created);
 
            }else
                setHead(created);
            map.put(key, created);
        }
    }
}