146. 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.

public class LRUCache {

    private int mSize;
    private int mCapacity;
    private Node start;
    private Node end;
    Map<Integer, Node> map = new HashMap<>();
    public LRUCache(int capacity) {
        mSize = 0;
        mCapacity = capacity;
        start = new Node();
        end = new Node();
        start.next = end;
        end.prev = start;

    }

    public int get(int key) {
        if(map.containsKey(key)){
            Node node = map.get(key);
            moveToHead(node);
            return node.val;
        }else{
            return -1;
        }
    }
    private void moveToHead(Node node){
        if(node.prev == start) return;

        node.prev.next = node.next;
        node.next.prev = node.prev;
        insertToHead(node);
    }
    private void insertToHead(Node node){
        Node next = start.next;
        node.next = next;
        next.prev = node;

        start.next = node;
        node.prev = start;


    }
    public void set(int key, int value) 
    {

        if(map.containsKey(key)){
            moveToHead(map.get(key));
            Node node = map.get(key);
            node.val = value;
        }else{
            Node node = new Node(key , value);
            if(mSize >= mCapacity){
                clearCache();
            }
            insertToHead(node); 
            mSize++;
            map.put(key, node);
        }


    }

    private void clearCache(){
        while(mSize >= mCapacity){
            Node tail = end.prev;
            if(tail == start) break;
            tail.prev.next = end;
            end.prev = tail.prev;

            map.remove(tail.key);
            mSize--;
        }
    }

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