我正在使用Java中的单链表实现deque。我的addLast（）无效

Question

我正在使用Java中的单链表实现deque。我的addFirst()功能正常，但addLast()无效。

每当我拨打addLast()时，都会收到以下错误消息：

  

显示java.lang.NullPointerException

Answer 1

你的上一次是npm install。

将其分配给import UIKit import Foundation func firstTask() -> Bool { // Do something print("First task") return true // change this to determine the task success or not } func secondTask() -> Bool { // Do something print("Second task") return true // change this to determine the task success or not } func thirdTask() { // Do something print("Third task") } let thirdBlock: (_ success: Bool) -> () = { success in guard success else { return } thirdTask() } let secondBlock: (_ success: Bool) -> () = { success in guard success else { return } let success = secondTask() thirdBlock(success) } let firstBlock: () -> () = { let success = firstTask() secondBlock(success) } firstBlock() 时，null也为空。

因此，当您致电old_last时，old_last会抛出。

Answer 2

为Node类提供构造函数将有助于保持代码简短和干燥：

private class Node {
  Item item;
  Node next;
  private Node(Item item, Node next) {
    if (item == null) throw new NullPointerException();
// 'this' refers to the created instance and helps distinguish the field from the param
    this.item = item;  
    this.next = next;
  }
}

public void addFirst(Item item) {
  // creates a new Node before first so to speak and then repoints first to this node 
  first = new Node(item, first);   
  if (num_elements==0) last = first;
  num_elements++;
}

public void addLast(Item item) {
  if (num_elements == 0) {  
    // this will deal with the case (last==null) which causes the NPE
    addFirst(item);
    return;
  }
  last.next = new Node(item, null);
  last = last.next;
  num_elements++;
}

除此之外，单链表不是双端队列的理想数据结构。在两端添加O(1)时，从后面删除O(N)！