leetcode之旅（9）-Reverse Linked List

题目描述：

Reverse a singly linked list.

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Hint:
A linked list can be reversed either iteratively or recursively. Could you implement both?

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分析：

注意这个数据结构，这是一个链表，要求颠倒顺序。考虑设置两个变量，来表示相邻的两个节点one和two，首先把头节点的next设置null，先取得three = two.next然后one= two.next。取得往后移动。one= two ，two= three

代码：

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