lru.java

import java.util.HashMap;

public class lru {
	final Node head = new Node(0, 0);
    final Node tail = new Node(0, 0);
    final int capacity;
    final HashMap<Integer, Node> map;
    
    public lru(int capacity) {
        this.capacity = capacity;
        map = new HashMap(capacity);
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int key) {
        int res = -1;
        if (map.containsKey(key)){
            Node n = map.get(key);
            remove(n);
            insertToHead(n);
            res = n.value;
        }
        return res;
    }
    
    public void put(int key, int value) {
        if (map.containsKey(key)) {
            Node n = map.get(key);
            remove(n);
            n.value = value;
            insertToHead(n);
        } else{
            if(map.size() == capacity){
                map.remove(tail.prev.key);
                remove(tail.prev);
            }
            Node n = new Node(key, value);
            insertToHead(n);
            map.put(key, n);
        }
    }
    
    private void remove(Node n){
        n.prev.next = n.next;
        n.next.prev = n.prev;
    }
    
    private void insertToHead(Node n){
        Node headNext = head.next;
        head.next = n;
        headNext.prev = n;
        n.prev = head;
        n.next = headNext;
    }
    
    class Node{
        Node prev, next;
        int key, value;
        Node(int k, int v){
            key = k;
            value = v;
        }
    }
}