【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; *         next = null; *     } * } */public class Solution {    public ListNode swapPairs(ListNode head) {        if( head==null )return null;if( head.next==null )return head;ListNode newHead = new ListNode(0);ListNode tail = newHead;ListNode first = null;ListNode second = null;while( head!=null && head.next!=null ){first = head;second = head.next;head = head.next.next;tail.next = second;second.next = first;tail = first;tail.next = null;}if( head!=null ){tail.next = head;tail = tail.next;}return newHead.next;       }}