【LeetCode】24. Swap Nodes in Pairs(Java)

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题目描述：给定一个链表，每交换两个相邻节点并返回其头部。

您不能修改列表节点中的值，只能修改节点本身。

Example:
Given 1->2->3->4, you should return the list as 2->1->4->3.

public static ListNode swapPairs(ListNode head) {
        if ((head == null) || (head.next == null))
            return head;
        ListNode Next = head.next;
        head.next = swapPairs(head.next.next);
        Next.next = head;
        return Next;
    }

public static ListNode swapPairs2(ListNode head) {
        if (head == null)
            return null;
        ListNode helper = new ListNode(0);
        helper.next = head;
        ListNode pre = helper;
        ListNode cur = head;
        while (cur != null && cur.next != null) {
            ListNode next = cur.next.next;
            cur.next.next = cur;
            pre.next = cur.next;
            if (next != null && next.next != null)
                cur.next = next.next;
            else
                cur.next = next;
            pre = cur;
            cur = next;
        }
        return helper.next;
    }