LRU

LRU.java

class LRUCache {
    private int capacity;
    private int occupied;
    
    private HashMap<Integer, Buffer> refs;
    private Buffer head;
    private Buffer tail;
    

    public LRUCache(int capacity) {
        this.capacity = capacity;
        occupied = 0;
        refs = new HashMap<>();
        head = new Buffer();
        tail = new Buffer();
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int key) {
        if (!refs.containsKey(key)) {
            return -1;
        }
        Buffer buf = refs.get(key);
        remove(buf);
        add(buf);
        return buf.val;
    }
    
    public void put(int key, int value) {
        if (refs.containsKey(key)) {
            Buffer buf = refs.get(key);
            buf.val = value;
            remove(buf);
            add(buf);
            return;
        }
        
        if (occupied >= capacity) {
            evict();
        }
        
        Buffer buf = new Buffer();
        buf.key = key;
        buf.val = value;
        add(buf);
    }
    
    private void add(Buffer buf) {
        buf.next = head.next;
        head.next.prev = buf;
        
        head.next = buf;
        buf.prev = head;
        refs.put(buf.key, buf);
        occupied++;
    }
    
    private void remove(Buffer buf) {
        buf.prev.next = buf.next;
        buf.next.prev = buf.prev;
        refs.remove(buf.key);
        occupied--;
    }
    
    private void evict() {
        remove(tail.prev);
    }
    
    private static class Buffer {
        Buffer prev;
        Buffer next;
        int key;
        int val;
    }
}

/**
 * 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);
 */