Implement an LRU Cache

In this article, I explained about LRU Cache implementation with example.

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.

The cache is initialized with a positive capacity.

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 – Java Program

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


class LRUCache {
    
    Node head = new Node(0,0);
    Node tail = new Node(0,0);
    int capacity;
    HashMap<Integer,Node> map = new HashMap<Integer,Node>();
    public LRUCache(int capacity) {
        this.capacity = capacity;
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int key) {
        if(map.containsKey(key))
        {
            Node node = map.get(key);
            remove(node);
            insert(node);
            return node.value;
        }
        else
            return -1;
    }
    
    public void put(int key, int value) {
        if(map.containsKey(key)){
            remove(map.get(key));
        }
        if(map.size() == capacity){
            remove(tail.prev);
        }
        insert(new Node(key,value));
    }
    
    public void remove(Node node)
    {
        map.remove(node.key);
        node.prev.next = node.next;
        node.next.prev = node.prev;
    }
    
    public void insert(Node node)
    {
        map.put(node.key,node);
        Node headNext = head.next;
        head.next = node;
        node.prev = head;
        headNext.prev = node;
        node.next = headNext;
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */

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