題目鏈接：https://leetcode-cn.com/problems/reverse-linked-list/
描述：反轉(zhuǎn)一個(gè)單鏈表。

示例:
輸入: 1->2->3->4->5->NULL
輸出: 5->4->3->2->1->NULL

代碼：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head == None or head.next == None:
            return head
        cur = head
        p = head.next
        pre = None
        while p:
            cur.next = pre
            pre = cur
            cur = p
            p = p.next
        cur.next = pre
        return cur