206. 反转链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null) return null;
        ListNode p = head.next,q;
        head.next=null;
        while(p!=null){      
            q = p.next;
            p.next = head;
            head = p;
            p = q;
        }
        return head;
    }
}