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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head==null||head.next==null) {
            return head;
        }
        ListNode pre,p;
        ListNode pHeadNext = head.next,pHead = head;
        //特殊处理前两个元素
        if (pHeadNext.next == null) {
            //只有2个元素
            pHeadNext.next = pHead;
            pHead.next = null;
            return pHeadNext;
        } else {
            pHead.next = pHeadNext.next;
            pHeadNext.next = pHead;
            //重新设置p pre
            p = pHead.next;
            pre = pHead;
        }
        while (p!=null&&p.next!=null) {
            ListNode pNext = p.next;
            pre.next = pNext;
            ListNode temp = pNext.next;
            pNext.next = p;
            p.next = temp;
            //重新设置p pre
            pre = p;
            p = pre.next;
        }
        return pHeadNext;
    }
}