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 listNode = new ListNode(0);
        listNode.next = head;
        ListNode p1 = listNode;
        ListNode p2 = head;
        while (p2 != null && p2.next != null) {
        	ListNode p3 = p2.next.next;
        	p2.next.next = p2;
        	p1.next = p2.next;
        	p2.next = p3;
        	p1 = p2;
        	p2 = p2.next;
        }
        return listNode.next;
    }
}