206. 反转链表

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

package p206;

/**
 * Created by admin on 2019/1/10.
 */
public class Main {

    public static class Node{
        public int value;
        public Node next;
        public Node(int data){
            this.value = data;
        }
    }

    public static void main(String[] args) {
        Node node1 = new Node(1);
        Node node2 = new Node(2);
        Node node3 = new Node(3);
        Node node4 = new Node(4);
        Node node5 = new Node(5);
        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node5;
        node5.next = null;

        Main m = new Main();
        Node node = m.reverseList2(node1);
        Node current = node;
        System.out.println(current.value);

    }

    public Node reverseList(Node head){
        Node pre = null;
        Node next = null;
        while (head != null) {
            next = head.next;
            head.next = null;
            pre = head;
            head = next;
        }
        return pre;
    }

    public Node reverseList2(Node head){
        if (head == null || head.next == null) {
            return head;
        }
        Node pre = null;
        while (head != null) {
            Node temp = head.next;
            head.next = pre;
            pre = head;
            head = temp;
        }

        return pre;
    }

}