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/**
 * 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 || head.next == null){
            return head;
        }
        ListNode toReturn = head;
        ListNode first = head;
        ListNode second = head.next;
        
        while(second != null){
            int temp = first.val;
            first.val = second.val;
            second.val = temp;
            if(second.next == null) return toReturn;
            first = first.next.next;
            second = second.next.next;
        }
        
        return toReturn;
    }
}

/**
 * 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(0);
        dummy.next = head;
        ListNode cur = dummy;
        while (cur.next != null && cur.next.next != null) {
            ListNode first = cur.next;
            ListNode second = cur.next.next;
            first.next = second.next;
            second.next = first;
            cur.next = second;
            cur = cur.next.next;
        }
        return dummy.next;
        
        
        

/**
 * 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 || head.next == null) return head;
        ListNode n = head.next;
        head.next = swapPairs(head.next.next);
        n.next = head;
        return n;
    }
}