雙端佇列

/**
 * 雙端佇列
*/
class Deque {
  constructor() {
    // 隊列當前索引
    this.count = 0
    // 隊頭索引
    this.lowestCount = 0
    // 儲存佇列
    this.items = {}
  }
  /**
   * 新增到隊頭
  */
  addFront(element) {
    if (this.isEmpty()) {
      this.addBack(element)
    } else if (this.lowestCount > 0) {
      this.lowestCount--
      this.items[this.lowestCount] = element
    } else {
      // 將所有元素向後移動一位
      for (let i = this.count; i > 0; i--) {
        this.items[i] = this.items[i - 1]
      }
      this.count++
      this.lowestCount = 0
      this.items[0] = element
    }
  }
  /**
   * 新增到隊尾
  */
  addBack(element) {
    this.items[this.count] = element
    this.count++
  }
  /**
   * 移除隊頭
  */
  removeFront() {
    if (this.isEmpty()) {
      return undefined
    } else {
      const res = this.items[this.lowestCount]
      delete this.items[this.lowestCount]
      this.lowestCount++
      return res
    }
  }
  /**
   * 移除隊尾
  */
  removeBack() {
    if (this.isEmpty()) {
      return undefined
    } else {
      this.count--
      const res = this.items[this.count]
      delete this.items[this.count]
      return res
    }
  }
  /**
   * 返回隊頭
  */
  peekFront() {
    if (this.isEmpty()) {
      return undefined
    } else {
      return this.items[this.lowestCount]
    }
  }
  /**
   * 新增到隊尾
  */
  peekBack() {
    if (this.isEmpty()) {
      return undefined
    } else {
      return this.items[this.count - 1]
    }
  }
  /**
   * 檢視佇列是否為空
  */
  isEmpty() {
    return this.count - this.lowestCount === 0
  }
  /**
   * 檢視佇列長度
  */
  size() {
    return this.count - this.lowestCount
  }
  /**
   * 清空佇列
  */
  clear() {
    this.count = 0
    this.lowestCount = 0
    this.items = {}
  }
}