LeetCode 206 链表 Reverse Linked List

LeetCode 206 链表 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?

代码：
iteratively

/**
 * 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 null;
        ListNode newNode = null;
        ListNode oldNode = head;
        while(oldNode != null){
            ListNode tmp = oldNode.next;
            oldNode.next = newNode;
            newNode = oldNode;
            oldNode = tmp;
        }
        return newNode;
    }
}

recursively

public ListNode reverseList(ListNode head) {
    return reverse(null,head);
}

private static ListNode reverse(ListNode pre,ListNode cur){
    if(cur==null) return pre;
    ListNode next = cur.next;
    cur.next = pre;
    return reverse(cur,next);
}

注意：
不要创建新节点