Swap Nodes in Pair

lc24. 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.

    public ListNode swapPairs(ListNode head) {
        if ( head == null) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode p1 = dummy;
        while(head != null && head.next != null){
            ListNode temp = head.next.next;
            head.next.next = head;
            p1.next = head.next;
            head.next = temp;
            p1 = head;
            head = temp;
        }

        return dummy.next;
    }