Leetcode 24 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; }
* }
*/
public class Solution {
public ListNode swapPairs(ListNode head) {
ListNode l = new ListNode(0);
ListNode l1,l2;
l1 = l;
while(head != null && head.next != null)
{
l1.next = head.next;
l2 = head;
head = head.next.next;
l1.next.next = l2;
l2.next = null;
l1 = l2;
}
l1.next = head;
return l.next;
}
}