实现 LRU 缓存置换算法 least-recently-used

class LRUCache {
  #map
  #length
  constructor(length) {
    this.#map = new Map()
    this.#length = length
  }
  get(key) {
    if (!this.#map.has(key)) {
      return
    }
    const value = this.#map.get(key)
    this.#map.delete(key)
    this.#map.set(key, value)
    return value
  }
  set(key, val) {
    if (this.#map.has(key)) {
      this.#map.delete(key)
    }
    if (this.#map.size > this.#length) {
      const firstKey = this.#map.keys().next().value
      this.#map.delete(firstKey)
    }
    this.#map.set(key, val)
  }
}