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import { ListNode } from "../reverse-linked-list/ListNode.ts";
export default function swapPairs(head: ListNode | null): ListNode | null {    if (!head) {        return null;    }    const res = new ListNode(0, head);    let pre: ListNode | undefined | null = res;    let cur: ListNode | undefined | null = pre?.next;    let next: ListNode | undefined | null = cur?.next;    while (pre && cur && next) {        cur.next = next.next;        next.next = cur;        pre.next = next;        pre = cur;
        cur = pre?.next;        next = cur?.next;    }    return res.next;}