Swap Nodes in Pairs

Swap Nodes in Pairs 

问题：

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.

思路：

　　画画图即可，加入几个first second dummy的变量就顺理成章了

我的代码：

public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null)    return head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode first = head;
        ListNode second = head.next;
        while(first != null && second != null)
        {
            ListNode next = second.next;
            pre.next = second;
            second.next = first;
            first.next = next;
            pre = first;
            if(next == null) return dummy.next;
            first = next;
            second = next.next;
        }
        return dummy.next;
    }
}