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 dummy = new ListNode(-1);

        dummy.next = head;
        ListNode p = dummy;
        while(p.next != null && p.next.next != null){
            ListNode next = p.next;
            ListNode next2 = p.next.next;
            p.next = next2;
            next.next = next2.next;
            next2.next = next;

            p = next;
        }

        return dummy.next;
    }
}