146. LRU Cache

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and put.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
put(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.

Follow up:
Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1);       // returns 1
cache.put(3, 3);    // evicts key 2
cache.get(2);       // returns -1 (not found)
cache.put(4, 4);    // evicts key 1
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

Solution:
hash map make the time of get() to O(1), doubleLinkedList make the node removal/add O(1).
1. Each time get/set put the access/new node to the head of the list.
            remove(node);
            moveToHead(node);
2. in set function, when the element is not in map and count larger than or equal the capacity, remove the least recently used element(LRU-remove the tail).don't forget to remove the node from map.

public class Ex146LRUCache {
 //https://www.geeksforgeeks.org/design-a-data-structure-for-lru-cache/
 //https://www.geeksforgeeks.org/design-a-data-structure-for-lru-cache/
 /*
  * linkedList不行吗，为什么一定要doublelinkedlist
  * 删除o(1)
  */
    public class DoubleLinkedNode{
        public DoubleLinkedNode pre, next;
        public int value;
        public int key;
        public DoubleLinkedNode(int key,int value){
            this.value = value;
            this.key = key;
        }
    }
    public DoubleLinkedNode head, tail;
    public HashMap<Integer,DoubleLinkedNode> map;
    public int capacity;
    public int count = 0;
    public Ex146LRUCache(int capacity) {
        head = new DoubleLinkedNode(0,0);
        tail = new DoubleLinkedNode(0,0);
        map = new HashMap<Integer,DoubleLinkedNode>();
        head.next = tail;
        tail.pre = head;
        this.capacity = capacity;
    }
    
    public int get(int key) {
        if(map.containsKey(key)){
            DoubleLinkedNode node = map.get(key);
            remove(node);
            moveToHead(node);
            return node.value;
        }else{
            return -1;
        }
    }
    
    public void put(int key, int value) {
        if(map.containsKey(key)){
            DoubleLinkedNode node = map.get(key);
            node.value = value;
            remove(node);
            moveToHead(node);
        }else{
            DoubleLinkedNode node = new DoubleLinkedNode(key,value);
            map.put(key, node);//important
            if(count >= capacity){
                map.remove(this.tail.pre.key);//delete map before remove the element from linkedList
                removeTail();
                moveToHead(node);
            }else{
                moveToHead(node);
            }
            count++;
        }
    }
    
    private void remove(DoubleLinkedNode node){
        DoubleLinkedNode pre = node.pre;
        DoubleLinkedNode next = node.next;
        pre.next = next;
        next.pre = pre;
        
    }
    private void removeTail(){
        DoubleLinkedNode last = this.tail.pre;
        DoubleLinkedNode lastPre = last.pre;
        lastPre.next = this.tail;
        this.tail.pre = lastPre;
    }    
    private void moveToHead(DoubleLinkedNode node){
        DoubleLinkedNode first = this.head.next;
        first.pre = node;
        node.next = first;
        this.head.next = node;
        node.pre = this.head;
    }
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  // case 1
//  Ex146LRUCache cache = new Ex146LRUCache(2);
//  cache.put(1, 1);
//  cache.put(2, 2);
//  System.out.println(cache.get(1));
//  cache.put(3, 3);
//  System.out.println(cache.get(2));
//  cache.put(4, 4);
//  System.out.println(cache.get(1));
//  System.out.println(cache.get(3));
//  System.out.println(cache.get(4));
  // case 2
  Ex146LRUCache cache = new Ex146LRUCache(1);
  cache.put(2, 1);
  System.out.println(cache.get(2));
  //["LRUCache","put","put","get","put","get","put","get","get","get"]
  //[[2],[1,1],[2,2],[1],[3,3],[2],[4,4],[1],[3],[4]]
 }