Swap Nodes in Pairs

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) {
            return head;
        }

        ListNode dummy = new ListNode(-1);
        dummy.next = head;

        ListNode current = dummy;

        while (current.next != null && current.next.next != null) {
            ListNode first = current.next;
            ListNode second = current.next.next;

            first.next = second.next;
            second.next = first;
            current.next = second;

            current = first;
        }

        return dummy.next;
    }
}

Hope this helps,
Michael