Reverse Linked List 解答

Question

Reverse a singly linked list.

Solution 1 -- Iterative

Remember to set head.next = null or it will report "memory limit exceeds" error.

 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 current = head, prev = head, post = head;
12         if (current == null || current.next == null)
13             return current;
14         current = current.next;
15         head.next = null;
16         while (current.next != null) {
17             post = current.next;
18             current.next = prev;
19             prev = current;
20             current = post;
21         }
22         current.next = prev;
23         return current;
24     }
25 }

Solution 2 -- Recursive

We can also use recursion to solve this problem.

 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)
12             return head;
13         ListNode second = head.next;
14         head.next = null;
15         ListNode newHead = reverseList(second);
16         second.next = head;
17         return newHead;
18     }
19 }