LeetCode Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

For example,
Given 1->2->3->4, you should return the list as 2->1->4->3.

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

public ListNode swapPairs(ListNode head) {
        //链表为空或者只有一个头节点
        if (head == null || head.next == null) {
            return head;
        }
        ListNode nHead = swapNode(head);
        //ptr指向已交换节点最后一个节点
        ListNode ptr = nHead.next;
        while (ptr.next != null && ptr.next.next != null) {
            ptr.next = swapNode(ptr.next);
            ptr = ptr.next.next;
        }
        return nHead;
    }

    /**
     * 交换2个ListNode
     * @param head
     * @return 返回交换后的第一个节点
     */
    private ListNode swapNode(ListNode head) {
        ListNode p = head;
        ListNode q = p.next;
        p.next = q.next;
        q.next = p;
        return q;
    }