算法 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 newHead=head.next;
        ListNode next=head.next;
        ListNode temp=null,temp2=null;
        while(head!=null&&next!=null)
        {
            temp=next.next;
            next.next=head;
            head.next=temp;
            if(temp2!=null)
            {
                temp2.next=next;
            }
            if(temp!=null&&temp.next!=null)
            {
                temp2=head;
                head=temp;
                next=temp.next;
            }
            else
            {
                break;
            }
            
        }
        return newHead;
        
    }
}