206.反转链表

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