合并两个有序链表错误点

版本1


/*
 * @lc app=leetcode.cn id=21 lang=javascript
 *
 * [21] 合并两个有序链表
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function (l1, l2) {
  const head = new ListNode(0, null);

  let p = head;
  while (l1 && l2) {
    const tmp = new ListNode(0, null);
    if (l1.val > l2.val) {
      tmp.val = l2.val;
      l2 = l2.next;
    } else {
      tmp.val = l1.val;
      l1 = l1.next;
    }
    p.next = tmp;
    p = p.next;
  }

  while (l1) {
    const tmp = new ListNode(0, null);

    tmp.val = l1.val;
    p.next = tmp;

    l1 = l1.next;
    p = p.next;
  }

  while (l2) {
    const tmp = new ListNode(0, null);

    tmp.val = l2.val;
    p.next = tmp;

    l2 = l2.next;
    p = p.next;

  }

  return head.next;
};
// @lc code=end

在版本1中，我们每次找到一个较小值都创建一个新的节点去获取val，实际上并没有必要，直接等于现有的节点，然后将节点断开连接

版本2


/*
 * @lc app=leetcode.cn id=21 lang=javascript
 *
 * [21] 合并两个有序链表
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function (l1, l2) {
  const head = new ListNode(0, null);

  let p = head;
  while (l1 && l2) {
    if (l1.val > l2.val) {
      p.next = l2;
      l2 = l2.next;
    } else {
      p.next = l1;
      l1 = l1.next;
    }
    p = p.next;
  }

  while (l1) {
    p.next = l1;

    l1 = l1.next;
    p = p.next;
  }

  while (l2) {
    p.next = l2;

    l2 = l2.next;
    p = p.next;

  }

  return head.next;
};
// @lc code=end

对于第一个while循环之后，判断l1,l2是否有剩余的这部分逻辑里，应当注意的是l1或者l2已经是有序排列的链表了，不需要再进行节点next的比较或者赋值，只需要让尾节点等于剩余部分的头节点就行了

最终版本


/*
 * @lc app=leetcode.cn id=21 lang=javascript
 *
 * [21] 合并两个有序链表
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function (l1, l2) {
  const head = new ListNode(0, null);

  let p = head;
  while (l1 && l2) {
    if (l1.val > l2.val) {
      p.next = l2;
      l2 = l2.next;
    } else {
      p.next = l1;
      l1 = l1.next;
    }
    p = p.next;
  }

  if (l1) {
    p.next = l1;
  }

  if (l2) {
    p.next = l2;
  }

  return head.next;
};
// @lc code=end