Monday, April 24, 2017

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
Thoughts:
The key to solve this problem is using a double linked list which enables us to quickly move nodes.
Java Solution:
public class LRUCache {
//define the double linked list
    private class Node {
        Node prev;
        Node next;
        int key;
        int value;
        public Node(int key, int value){
            this.key = key;
            this.value = value;
            this.prev = null;
            this.next = null;
        }
    }
    private int capacity;
    private HashMap hs = new HashMap();
    private Node head = new Node(-1, -1);
    private Node tail = new Node(-1, -1);
    public LRUCache(int capacity) {
        this.capacity = capacity;
        tail.prev = head;
        head.next = tail;
    }
    
    public int get(int key) {
        if(!hs.containsKey(key)){
            return -1;
        }
        
        //remove current
        Node current = hs.get(key);
        current.prev.next = current.next;
        current.next.prev = current.prev;
        
        //move current to tail; Meaning it has been recently used.
        move_to_tail(current);
        return hs.get(key).value;
    }
    
    public void put(int key, int value) {
        // this get method will update the key's position. Make sure it will not duplicated in LinkedList.
        if(get(key) != -1){
            hs.get(key).value = value;
            return;
        }
        //remove the least recently used key if reaching the capacity
        if(hs.size() == capacity){
            hs.remove(head.next.key);
            head.next = head.next.next;
            head.next.prev = head;
        }
        //insert the key, value if not reach the capacity
        Node insert = new Node(key, value);
        hs.put(key, insert);
        //move current to tail; Meaning it has been recently used.
        move_to_tail(insert);
        
    }
    private void move_to_tail(Node current){
        current.prev = tail.prev;
        tail.prev = current;
        current.prev.next = current;
        current.next = tail;
    }
}

/**
 * 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);
 */

No comments:

Post a Comment