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;
    }
}