Swap Nodes In Pairs

Given a linked list, swap every two adjacent nodes and return its head.

“You may not modify the values in the list’s nodes, only nodes itself may be changed.l”

Solution1

/ **
    l1 -> 1 -> 2 -> 3 -> 4
          |
          v
          l2
    
to 

    2 -> 1 -> 3 -> 4
         |    |
         v    v
         l1   l2
*/

public ListNode swapPairs(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }

    ListNode dummy = new ListNode(0);
    dummy.next = head;
    ListNode l1 = dummy;
    ListNode l2 = head;

    while (l2 != null && l2.next != null) {
        ListNode nextStart = l2.next.next;
        l1.next = l2.next;
        l2.next.next = l2;
        l2.next = nextStart;
        l1 = l2;
        l2 = l2.next;
    }
    return dummy.next;
}

Solution2

[Java] recursive solution beats 100% with explanation

public ListNode swapPairs(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }

    ListNode tempNext = head.next.next;
    ListNode tempFirst = head.next;
    ListNode tempSecond = head;
    head = tempFirst;
    head.next = tempSecond;
    head.next.next = swapPairs(tempNext);
    return head;
}