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;
    }
}