【leetcode链表】反转链表

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

非递归实现：

/**
 * 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 reversedHead = null;
        ListNode temp = head;
        ListNode pre = null;
        
        while(temp!=null) {
            ListNode next = temp.next;
            if(next==null) {
                //到尾部了
                reversedHead = temp;
            }
            
            temp.next = pre;
            pre = temp;
            temp = next;
            
        }
        
        return reversedHead;

    }
}

递归实现：

/**
 * 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;
        }
        
        ListNode node = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        return node;        

    }
}