leetcode 206. Reverse Linked List

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:

A linked list can be reversed either iteratively or recursively. Could you implement both?

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    //每次循环next节点时 用next节点val创建新节点 更改节点指针
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
		    return null;
	    }
        ListNode newHead = new ListNode(head.val);
        ListNode sub = head.next;
        while (sub != null) {
            ListNode node = newHead;
            //创建新的节点 val是next节点的val
            ListNode node1 = new ListNode(sub.val);
            //新节点的next指针 指向前一个节点
            node1.next = node;
            //新节点赋给newHead
            newHead = node1;
            sub = sub.next;
        }
        return newHead;
        
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode pre = null;
        ListNode cur = head;
        ListNode next = head.next;
        
        while (cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        
        return pre;
        
    }
}