Reverse a singly linked list.

Hint:

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

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Solving method:

class ListNode(object):
     def __init__(self, x):
         self.val = x
         self.next = None
class Solution(object):
    def reverseList(self, head):
        result = ListNode(None)
        while head:
            next = head.next
            head.next = result.next
            result.next = head
            head = next
        return result.next