迭代法
    时间复杂度 O（n）

    1. # 反转一个单链表。 
    2. # 
    3. #  示例: 
    4. # 
    5. #  输入: 1->2->3->4->5->NULL
    6. # 输出: 5->4->3->2->1->NULL 
    7. # 
    8. #  进阶: 
    9. # 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？ 
    10. #  Related Topics 链表 
    11. #  👍 1170 👎 0
    13. class ListNode(object):
    14.     def __init__(self, x):
    15.         self.val = x
    16.         self.next = None
    17. # leetcode submit region begin(Prohibit modification and deletion)
    18. # Definition for singly-linked list.
    19. # class ListNode(object):
    20. #     def __init__(self, x):
    21. #         self.val = x
    22. #         self.next = None
    24. class Solution(object):
    25.     def reverseList(self, head):
    26.         """
    27.         :type head: ListNode
    28.         :rtype: ListNode
    29.         """
    30.         pre = None
    31.         curr = head
    32.         while curr:
    33.             tmp = curr.next
    34.             curr.next = pre
    35.             pre = curr
    36.             curr = tmp
    37.         return pre
    39. # leetcode submit region end(Prohibit modification and deletion)