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 left = dummy;
        ListNode right = head;
        while (right != null && right.next != null) {
            ListNode nextHeader = right.next.next;
            right.next.next = right;
            left.next = right.next;
            right.next = nextHeader;
            left = right;
            right = nextHeader;
            
        }
        return dummy.next;
    }
}
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