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Approach: a Map keeps keys in insertion order, so its first key is always the least recently used. Re-insert a key on every get and put; when over capacity, evict the first key.

class LRUCache {
  constructor(capacity) {
    this.capacity = capacity;
    this.map = new Map(); // oldest key first
  }
  get(key) {
    if (!this.map.has(key)) return -1;
    const value = this.map.get(key);
    this.map.delete(key); this.map.set(key, value);
    return value;
  }
  put(key, value) {
    this.map.delete(key); this.map.set(key, value);
    if (this.map.size > this.capacity)
      this.map.delete(this.map.keys().next().value);
  }
}

Complexity: Time O(1) for each get and put · Space O(capacity).