近幾天為了測試自己自學的成果，所以上 LeetCode 練習解題。原以為自己在前面的題目已經基本熟悉了 Linked List 的操作，但是到了這題卻又出現意想不到的錯誤。

先附上程式碼：

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
  let nextNode = head.next;
  let currNode = head;
  let prevNode = null;
  while (true) {
    currNode.next = prevNode;
    currNode = nextNode;
    nextNode = nextNode.next;
    prevNode = currNode;
    if (!nextNode) { return currNode }
  }
};

錯誤訊息：

Line 13 in solution.js
  let nextNode = head.next;
                      ^
TypeError: Cannot read properties of null (reading 'next')
    Line 13: Char 23 in solution.js (reverseList)
    Line 33: Char 19 in solution.js (Object.<anonymous>)
    Line 16: Char 8 in runner.js (Object.runner)
    Line 24: Char 26 in solution.js (Object.<anonymous>)
    at Module._compile (node:internal/modules/cjs/loader:1101:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47

head 明明是 list 的頭，為什麼不能接 .next？

先前還嘗試過另一種寫法：

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
  let currNode = head;
  let prevNode; // 這裡很奇怪
  while (true) {
    const fixedCurrNode = currNode;
    currNode = currNode.next;
    fixedCurrNode.next = prevNode;
    prevNode = fixedCurrNode;
    if (!currNode) { return fixedCurrNode }
  }
};

如果 prevNode 最開始沒有賦值（undefined），就會報以下型別錯誤：

Line 37 in solution.js
             throw new TypeError(__serialize__(ret) + " is not valid value for the expected return type ListNode");
             ^
TypeError: {"val":5,"next":{"val":4,"next":{"val":3,"next":{"val":2,"next":{"val":1,"next":undefined}}}}} is not valid value for the expected return type ListNode
    Line 37: Char 20 in solution.js (Object.<anonymous>)
    Line 16: Char 8 in runner.js (Object.runner)
    Line 23: Char 26 in solution.js (Object.<anonymous>)
    at Module._compile (node:internal/modules/cjs/loader:1101:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47

但如果在開始時就設 prevNode = null，就會跳出跟第一種寫法相似的錯誤：

Line 17 in solution.js
    currNode = currNode.next;
                        ^
TypeError: Cannot read properties of null (reading 'next')
    Line 17: Char 25 in solution.js (reverseList)
    Line 32: Char 19 in solution.js (Object.<anonymous>)
    Line 16: Char 8 in runner.js (Object.runner)
    Line 23: Char 26 in solution.js (Object.<anonymous>)
    at Module._compile (node:internal/modules/cjs/loader:1101:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)
    at Module.load (node:internal/modules/cjs/loader:981:32)
    at Function.Module._load (node:internal/modules/cjs/loader:822:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    at node:internal/main/run_main_module:17:47

我去 Solution 區看了一些別人寫的範例，看起來都跟我大同小異，但是卻沒有這種錯誤，在 Google 上也都只搜尋到誤當成 Array 操作的案例，請問各位前輩能不能指點一下修正的方向？