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import { ListNode } from "../reverse-linked-list/ListNode.ts";export default function mergeTwoLists(    list1: ListNode | null,    list2: ListNode | null,): ListNode | null {    if (!list1) return list2;    if (!list2) return list1;    let l1: ListNode | null = list1;    let l2: ListNode | null = list2;    const prehead = new ListNode(-1);    let prev = prehead;    while (l1 != null && l2 != null) {        if (l1.val <= l2.val) {            prev.next = l1;            l1 = l1.next;        } else {            prev.next = l2;            l2 = l2.next;        }        prev = prev.next;    }    prev.next = l1 === null ? l2 : l1;
    return prehead.next;}