206. Reverse Linked List

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

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