24 Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
// 1 -> 2 -> 3 -> 4 -> null
//     5 <-4 <- 3<- 1 <- 2 
//              l2   l1 
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode l1 = dummy;
        ListNode l2 = head;
        while(l2 != null && l2.next != null) {
            ListNode nextHead = l2.next.next;//points to 3
            l1.next = l2.next; 
            l2.next.next = l2;
            l2.next = nextHead;
            l1 = l2;
            l2 = l2.next;
        }
        return dummy.next;
    }
}