【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;
}
}