Reverse Linked List

Reverse a singly linked list.

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Hint:

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

 

Iteratively:

 1 /**

 2  * Definition for singly-linked list.

 3  * public class ListNode {

 4  *     int val;

 5  *     ListNode next;

 6  *     ListNode(int x) { val = x; }

 7  * }

 8  */

 9 public class Solution {

10     public ListNode reverseList(ListNode head) {

11         ListNode first = null;

12         ListNode second = head;

13         while(second != null){

14             ListNode tmp = second.next;

15             second.next = first;

16             first = second;

17             second = tmp;

18         }

19         return first;

20     }

21 }

 

recursively:

 1 /**

 2  * Definition for singly-linked list.

 3  * public class ListNode {

 4  *     int val;

 5  *     ListNode next;

 6  *     ListNode(int x) { val = x; }

 7  * }

 8  */

 9 public class Solution {

10     public ListNode reverseList(ListNode head) {

11         if(head == null || head.next == null) return head;

12         ListNode nextNode = head.next;

13         head.next = null;

14         ListNode newHead = reverseList(nextNode);

15         nextNode.next = head;

16         return newHead;

17     }

18 }