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.
** 解题思路** Using Recursion and Iterative Solution

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// Iterative
   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;
           cur.next = second;
           cur.next.next = first;
           cur = cur.next.next;
           
       }
       return dummy.next;
   }

  // Recursion
   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;
   }