LeetCode--No.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) {
if (head == null || head.next == null)
return head;
ListNode left = head;
ListNode right = head.next;
ListNode last = right.next;
left.next = last;
right.next = left;
head = right;
ListNode prev = left;
while(last != null && last.next != null){
left = last;
right = last.next;
last = right.next;
prev.next = right;
right.next = left;
left.next = last;
prev = left;
}
return head;
}
}