反向链接列表没有头

Question

我有一个反向链接列表，该列表不返回头。列表打印出来。但是，当我在其他函数中使用它时，我无法访问countList函数中链表的开头。我试过使用head.prev，但这也不起作用。我正在尝试使用反向链接列表的节点中的所有字母创建一个字符串。

class Node{
  constructor(data, next = null){
    this.data = data;
    this.next = next;
  }
}

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

  insertLast(value){
    if(this.head === null){
      this.head = new Node(value);
    }else{
        let node = this.head;
        while(node.next){
          node = node.next;
        }
        node.next = new Node(value);
      }
    }
}//constructor end

let listOne = new LinkedList();
let listTwo = new LinkedList();
listOne.insertLast(5);
listOne.insertLast(4);
listOne.insertLast(2);
listTwo.insertLast(7);
listTwo.insertLast(9);
listTwo.insertLast(1);
console.log(listOne);

const countList = function(list){
  console.log('here is the list in coutnlist', list);
  let curr = list.head;
  console.log("this is curr ", curr);
  // let str = '';
  while(curr.next){
    curr = curr.next;
    str+=curr.data;
  }
  return str;
}

const sumLists = function(listOne, listTwo){
  let reverse1 = reverse(listOne);
  console.log('thisi s reverse 1', reverse1)
  let reverse2 = reverse(listTwo);
  let firstNum = countList(reverse1);
  let secondNum = countList(reverse2);
  console.log('this is first num', firstNum);
  let finalNum = parseInt(firstNum + secondNum);
  return finalNum;
}

const reverse = function(list){
  let curr = list.head;
  let prev = null;
  let temp = null;
  while(curr){
    temp = curr.next;
    curr.next = prev;
    prev = curr;
    curr = temp;
  }
  return prev;
}
console.log(sumLists(listOne, listTwo));