How would you implement an LRU cache with O(1) get and put?

HashMap + Doubly Linked List

class LRUCache<K, V> {
  private map = new Map<K, Node<K,V>>();
  constructor(private capacity: number) {}

  get(key: K): V | null {
    const node = this.map.get(key);
    if (!node) return null;
    this.moveToHead(node); // Mark recently used
    return node.value;
  }

  put(key: K, value: V): void {
    if (this.map.has(key)) { /* update + move to head */ return; }
    const node = new Node(key, value);
    this.map.set(key, node);
    this.addToHead(node);
    if (this.map.size > this.capacity) {
      const lru = this.removeTail(); // Evict least recently used
      this.map.delete(lru.key);
    }
  }
}

All operations O(1): HashMap for lookup, linked list for ordering.