leetcode--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. 

Have you been asked this question in an interview? 

The main point of this problem is to guarantee each operation cost O(1) time, This is the main requirement of cache.

Since we need to remove a record from the cache and keep all records in order,  this invloves searching, deletion and insertion. The best data structure to do this is the

one which is a combination of HashMap and doubly-linked list. So, I use a HashMap<Integer, Node>, where Node is class defined by myself.

public class LRUCache {
   public LRUCache(int capacity) {
        lruMap = new HashMap<Integer, Node>();
		this.capacity = capacity;
		sz = 0;		
    }
    
    public int get(int key) {
        int result = -1;
		if(lruMap.containsKey(key)){
			moveToHead(lruMap.get(key));
			lruMap.put(key, first);
			result = first.value;
		}
		return result;
    }
    
    public void set(int key, int value) {
        if(lruMap.containsKey(key)){
			Node temp = lruMap.get(key);
			temp.value = (temp.value == value )? temp.value : value;
			moveToHead(temp);
			lruMap.put(key, first);
		}
		else if(sz < capacity){
			Node node = new Node();
			node.key = key;
			node.value = value;
			if(sz == 0){
				first = node;
				last = node;
			}
			else{
				first.prev = node;
				node.next = first;
				first = node;
			}
			lruMap.put(key, node);
			++sz;
		} 
		else{
			Node node = new Node();
			node.key = key;
			node.value = value;
			int lastKey = last.key;
			if(sz == 1){
				last = null;
				first = null;
				first = node;
				last = node;
				
			}
			else{
				last = last.prev;
				last.next = null;
				first.prev = node;
				node.next = first;
				first = node;				
			}
			lruMap.remove(lastKey);
			lruMap.put(key, node);			
		}
    }
	
	public  void moveToHead(Node node){
		if( node.prev == null)   //the node is already the head. Do nothing
			return;
		else{
			node.prev.next = node.next;
			if(node.next != null)
				node.next.prev = node.prev;
			else
				last = node.prev;
			first.prev = node;
			node.prev = null;
			node.next = first;
			first = node;			 
		}
	}
	
	private HashMap<Integer, Node> lruMap;
	private int capacity;
	private Node first;
	private Node last;
	private int sz;
}

class Node{
	int key; //we need use it to remove a node in the HashMap;
	int value;
	Node prev;
	Node next;
	//using the default constructor is enough
}