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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null) return null;
		if(head.next==null) return head;
		ListNode first = new ListNode(-1);
		first.next = head;
		ListNode p = first;
		
		ListNode pre = p;
	    ListNode mid = p.next;
	    ListNode last = mid==null?null:mid.next;	    
	    while(mid!=null && last!=null){
	    	mid.next = last.next;
	    	last.next = mid;
	    	pre.next = last;
	    	
	    	pre = pre.next.next;
	    	mid = pre.next==null?null:mid.next;
	    	last = mid==null?null:mid.next;
	    }	    
	    return first.next;
    }
}