leetcode hot 11

解题思路:熟能生巧，链表问题就是用指针来模拟，链表反转很经典，利用三个指针，两个指着head，一个指着null，然后模拟。

/**
 * 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) {
        if(head==null) return head;
        ListNode pre = null;
        ListNode cur = head;
        ListNode next = head;
        while(cur!=null){
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}