算法题：链表反转

 1.使用栈

class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
            next = null;
        }

    }

public ListNode ReverseList (ListNode head) {
        /**
         * 使用栈
         */
        if (head == null){
            return null;
        }
        // write code here
        Stack nodes = new Stack<>();
        ListNode pos = head;
        while (pos!=null){
            nodes.push(pos);
            pos = pos.next;
        }
        ListNode newHead = nodes.pop();

        pos = newHead;
        while (!nodes.empty()){
            pos.next = nodes.pop();
            pos = pos.next;
        }
        pos.next = null;
        return newHead;
  }

2.使用双链表

public ListNode ReverseList2 (ListNode head) {
        /**
         * 双链表
         */
        if (head == null){
            return null;
        }
        //定义一个新链表
        ListNode newHead = null;
        while (head!=null){
            ListNode temp = head.next;
            head.next = newHead;
            newHead = head;
            head = temp;
        }
        return newHead;
    }

3.使用递归

public ListNode ReverseList3 (ListNode head) {
        /**
         * 递归
         */
        if (head == null || head.next == null){
            return head;
        }

        ListNode next = head.next;
        ListNode listNode = ReverseList3(next);
        next.next = head;
        head.next = null;
        return listNode;
    }