队列实现

class ListNode<T> {
  val: T | null
  prev: ListNode<T> | null
  next: ListNode<T> | null
 
  constructor(val?: T, prev?: ListNode<T>, next?: ListNode<T>) {
    this.val = val || null
    this.prev = prev || null
    this.next = next || null
  }
}
 
class Queue<T> {
  private head: ListNode<T> | null = null
  private tail: ListNode<T> | null = null
  private len = 0
 
  enqueue(item: T) {
    const node = new ListNode(item)
    if (this.head == null || this.tail == null) {
      this.head = this.tail = node
    } else {
      this.tail.next = node
      node.prev = this.tail
      this.tail = node
    }
    this.len++
  }
 
  dequeue() {
    if (this.head == null) return null
    const val = this.head.val
    this.head = this.head.next
    if (this.head != null) this.head.prev = null
    this.len--
    return val
  }
 
  isEmpty() {
    return this.head == null
  }
 
  size() {
    return this.len
  }
 
  front() {
    if (this.head == null) return null
    return this.head.val
  }
 
  clear() {
    this.head = null
    this.tail = null
    this.len = 0
  }
}