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.

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

Solution 2 -- Recursive

We can also use recursion to solve this problem.

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