删除最后一个元素后未定义父节点

class ListNode {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class LinkedList {
  constructor() {
    this.head = null;
  }

  append(value) {
    const newNode = new ListNode(value);
    if (!this.head) {
      this.head = newNode;
    } else {
      let current = this.head;
      while (current.next) {
        current = current.next;
      }
      current.next = newNode;
    }
  }

  deleteLast() {
    if (!this.head) return;

    if (!this.head.next) {
      this.head = null;
    } else {
      let current = this.head;
      while (current.next.next) {
        current = current.next;
      }
      current.next = null;
    }
  }
}

// 示例使用
const list = new LinkedList();
list.append(1);
list.append(2);
list.append(3);

console.log("Before deletion:");
let current = list.head;
while (current) {
  console.log(current.value);
  current = current.next;
}

list.deleteLast();

console.log("After deletion:");
current = list.head;
while (current) {
  console.log(current.value);
  current = current.next;
}