0024-swap-nodes-in-pairs.java

//Iterative version
class Solution {

    public ListNode swapPairs(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode temp = dummy;
        while (temp.next != null && temp.next.next != null) {
            ListNode first = temp.next;
            ListNode second = temp.next.next;
            temp.next = second;
            first.next = second.next;
            second.next = first;
            temp = first;
        }
        return dummy.next;
    }
}

//Recursive version
class Solution {

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