206. 反转链表

206. 反转链表

一个头插法干掉

/**
 * 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 dummy = new ListNode();
        ListNode p = head, r;
        while(p!=null){
            r = p.next;
            p.next = dummy.next;
            dummy.next = p;
            p = r;
        }
        return dummy.next;
    }
}