三指针, cur是当前要反转的节点 next在反转前移动

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    /**
     * 三指针, cur是当前要反转的节点
     * next在反转前移动
     */
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null)
            return head;
        // prev(-1) cur(0) next(0)
        ListNode prev = null, cur = head, next = head;
        while (cur != null) {
            next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }
        return prev;
    }
}