[LeetCode] Problem 146 - LRU Cache

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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

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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

Code

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public class LinkedNode {
    private int key;
    private int value;
    private LinkedNode prev;
    private LinkedNode next;

    public LinkedNode(int key, int value) {
        this.key = key;
        this.value = value;
    }
}

private int capacity;
private Map<Integer, LinkedNode> map;
LinkedNode head;
LinkedNode tail;

public LRUCache(int capacity) {
    this.capacity = capacity;
    this.map = new HashMap<>();
    this.head = new LinkedNode(-1, -1);
    this.tail = new LinkedNode(-1, -1);

    head.next = tail;
    tail.prev = head;
}

public int get(int key) {
    if (!map.containsKey(key))
        return -1;

    LinkedNode node = map.get(key);
    remove(node);
    add(node);
    return node.value;
}

public void put(int key, int value) {
    if (map.containsKey(key)) {
        LinkedNode node = map.get(key);
        node.value = value;

        remove(node);
        add(node);
    } else {
        if (map.size() >= capacity) {
            LinkedNode last = tail.prev;
            remove(last);
            map.remove(last.key);
        }

        LinkedNode node = new LinkedNode(key, value);
        add(node);
        map.put(key, node);
    }
}

private void remove(LinkedNode node) {
    node.prev.next = node.next;
    node.next.prev = node.prev;
}

private void add(LinkedNode node) {
    node.next = head.next;
    head.next.prev = node;
    head.next = node;
    node.prev = head;
}