146. LRU Cache (Hard)

https://leetcode.com/problems/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

Solutions

1.

class LRUCache {

    int capacity = 0;
    int version = 0;

    Map<Integer, Integer> versionMap = new HashMap<>();
    Map<Integer, Integer> lruCache = new HashMap<>();
    Queue<Integer> versionQueue = new PriorityQueue<>(Comparator.comparingInt(o -> versionMap.get(o.intValue())));

    public LRUCache(int capacity) {
        this.capacity = capacity;
    }

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

        updateVersion(key);

        return lruCache.get(key);
    }

    public void put(int key, int value) {
        if (lruCache.containsKey(key)) {
            lruCache.put(key, value);

            updateVersion(key);

            return;
        }

        // if exceeds the capacity of queue, remove least recent used element at first
        if (versionMap.size() >= capacity) {
            removeLRU();
        }

        version++;

        lruCache.put(key, value);
        versionMap.put(key, version);
        versionQueue.add(key);
    }

    private void updateVersion(Integer key) {
        version++;

        // update version
        versionMap.put(key, version);

        // resort version queue
        versionQueue.remove(key);
        versionQueue.add(key);
    }

    private void removeLRU() {
        int k = versionQueue.poll();

        lruCache.remove(k);
        versionMap.remove(k);
        versionQueue.remove(k);
    }
}

2.

Incorrect Solutions

References