反转单链表

1. 非递归 - 无头结点

        /// <summary>
        /// 非递归 - 无头结点
        /// </summary>
        /// <param name="head"></param>
        public static ListNode ReverseList(ListNode head)
        {
            if (head == null)
                return null;

            ListNode newHead = head;
            ListNode temp =null;

            head = head.Next;
            newHead.Next = null;

            while (head != null)
            {
                temp = newHead;
                newHead = head;
                head = head.Next;
                newHead.Next = temp;
            }

            return newHead;
        }

2. 非递归 - 有头结点

        /// <summary>
        /// 非递归 - 有头结点
        /// </summary>
        /// <param name="head"></param>
        public static ListNode ReverseList(ListNode head)
        {
            if (head == null)
                return null;            

            ListNode newHead = new ListNode(head.Value);
            ListNode temp =null;

            while (head.Next != null)
            {
                temp = newHead.Next;
                newHead.Next = head.Next;
                head.Next = head.Next.Next;
                newHead.Next.Next = temp;
            }

            return newHead;
        }

3. 递归 - 无头结点

        /// <summary>
        /// 递归
        /// </summary>
        /// <param name="head"></param>
        public static ListNode ReverseList(ListNode head)
        {
            if (head == null || head.Next == null)
                return head;

            ListNode newHead = ReverseList(head.Next);

            head.Next.Next = head;
            head.Next = null;

            return newHead;
        }