solution.ts

/*
 * @lc app=leetcode id=21 lang=javascript
 *
 * [21] Merge Two Sorted Lists
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

interface ListNode {
  val: number;
  next: ListNode | null;
}

/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
const mergeTwoLists = (l1: ListNode | null, l2: ListNode | null): ListNode => {
  // * ['60 ms', '86.75 %', '35.5 MB', '79.49 %']

  const head: ListNode = { val: 0, next: null };

  let cur = head;

  while (l1 && l2) {
    if (l1.val < l2.val) {
      cur.next = l1;
      l1 = l1.next;
    } else {
      cur.next = l2;
      l2 = l2.next;
    }

    cur = cur.next;
  }

  // * one list clear, one list remaining
  cur.next = l1 || l2;

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

export { mergeTwoLists };