笨方法

/**
 * 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 {
    public ListNode reverseList(ListNode head) {
        ListNode ans = null;
        for (ListNode x = head; x != null; x = x.next) {
            ans = new ListNode(x.val, ans);
        }
        return ans;
    }
}

其他还有点懵，之后看