[LeetCode] 206. Reverse Linked List ☆(反转链表)

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

public static ListNode reverseLinkedList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode node = reverseLinkedList(head.next);
        head.next.next = head;
        head.next = null;
        //比如1->2->3，第一次执行到这里：3->2->NULL
        return node;
    }