逆序，想到头插法：

/**
 * 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 p;
        ListNode q;
        p = null;
        q = head;
        while (q!=null) {
            ListNode s;
            s = q.next;
            q.next = p;
            p = q;
            q = s;
        }
        return p;  
    }
}