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.

The cache is initialized with a positive capacity.

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

Idea: custom Linked Hash Map based on Map with defined head and tail nodes.
*/

class LRUCache {
    private final int capacity;
    private HashMap<Integer, Integer> map;
    private LinkedHashSet<Integer> set;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.map = new HashMap<>(capacity);
        this.set = new LinkedHashSet<>(capacity);
    }

    public int get(int key) {
        int value = map.getOrDefault(key, -1);
        if (value != -1) {
            updateOrder(key);
        }
        return value;
    }

    public void put(int key, int value) {
        if (map.size() >= capacity && !map.containsKey(key)) {
            Integer lastKey = set.stream().findFirst().get();
            set.remove(lastKey);
            map.remove(lastKey);
        }

        map.remove(key);
        map.put(key, value);
        updateOrder(key);
    }

    private void updateOrder(int key) {
        set.remove(key);
        set.add(key);
    }
}