206 反转链表

方法一：迭代法

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

方法二：递归法

/**
 * 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||head.next==null) return head;
        else{
         ListNode newHead=reverseList(head.next);
            head.next.next=head;
            head.next=null;
            return newHead;
        }
    }
}