0206. Reverse Linked List

https://leetcode.com/problems/reverse-linked-list

Description

Given the head of a singly linked list, reverse the list, and return the reversed list.

Example 1:

**Input:** head = [1,2,3,4,5]
**Output:** [5,4,3,2,1]

Example 2:

**Input:** head = [1,2]
**Output:** [2,1]

Example 3:

**Input:** head = []
**Output:** []

Constraints:

• The number of nodes in the list is the range [0, 5000].

• -5000 <= Node.val <= 5000

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

ac1: iterative

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

        ListNode prev = null, curr = head, next = curr.next;

        while (curr.next != null) {
            curr.next = prev;
            prev = curr;
            curr = next;
            next = next.next;
        }

        return prev;
    }
}

ac2: recursive

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

        ListNode tmp = reverseList(head.next);
        head.next.next = head;
        head.next = null;

        return tmp;
    }
}