LeetCode Swap Nodes in Pairs

递归实现：(思路非常清晰)

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

非递归实现：

假如现在是 1->2->3->4->5->6

  public ListNode swapPairs(ListNode head) {    if(head==null || head.next==null){        return head;    }     ListNode newHead = head.next;    // a = 1, b = 2     ListNode a=head, b=a.next, pre = null;    while(a!=null && b!=null){    // a.next = 3 ,b.next = 1      a.next = b.next;      b.next = a;      if(pre!=null){          pre.next = b;        }       if(a.next==null) break;  // 如果是奇数，下一个为空就会跳出循环，否则就会执行下面三条语句，进行交换      // b = 4, pre = 1, a = 3      b = a.next.next;      pre = a;      a = a.next;    }    return newHead;  }