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.

Solution: Data structure: Hashmap. When we operate the element, it will change the position in the linkedlist.

public class Solution {
    int capacity;
    HashMap<Integer, Node> map;
    Node head = null;
    Node end = null;
    // @param capacity, an integer
    public Solution(int capacity) {
        // write your code here
        this.capacity = capacity;
        map = new HashMap<Integer, Node>();
    }

    // @return an integer
    public int get(int key) {
        // write your code here
        if(map.containsKey(key)) {
            Node n = map.get(key);
            remove(n);
            setHead(n);
            return map.get(key).val;
        }
        return -1;
    }

    // @param key, an integer
    // @param value, an integer
    // @return nothing
    public void set(int key, int value) {
        // write your code here
        if(map.containsKey(key)) {
            Node node = map.get(key);
            node.val = value;
            map.put(key, node);
            remove(node);
            setHead(node);
        }
        else {
            if(map.size() < capacity) {
                Node n = new Node(key, value);
                map.put(key, n);
                setHead(n);
            }
            else {
                //must remove hashmap first. Because remove(end) will change the end pointer.
                map.remove(end.key);
                remove(end);
                Node n = new Node(key, value);
                map.put(key, n);
                setHead(n);  
            }
        }
    }

    public void setHead(Node n) {
        if(head == null) {
            //Initialization 
            head = n;
            end = n;
        }
        else {
            n.next = head;
            head.prev = n;
            head = n;
        }
    }

    public void remove(Node n) {
        if(head.equals(n)) {
            head = n.next;
            n.next = null;
        }
        else if(end.equals(n)) {
            end = n.prev;
            n.prev = null;
        }
        else {
            n.next.prev = n.prev;
            n.prev.next = n.next;
            n.next = null;
            n.prev = null;
        }
    }
}

class Node {
    int key;
    int val;
    Node prev;
    Node next;
    public Node(int key, int val) {
        this.key = key;
        this.val = val;
    }
}