java 交換鏈表中所有相鄰結點的順序

題目：

 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. 

算法如下：

/**
 * 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 newHead = head.next;
        while(head.next!=null){
            ListNode nextNode = head.next;
            if(nextNode.next==null){
                head.next = null;
            }else{
                if(nextNode.next.next==null)
                    head.next = nextNode.next;
                else
                    head.next = nextNode.next.next;
            }
            ListNode temp = head;
            head = nextNode.next;
            nextNode.next = temp;
            if(head==null){
                break;
            }
        }
        return newHead;
    }
}