206. Reverse Linked List

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

class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        prev = None
        current = head
        while current:
            next = current.next
            current.next = prev
            prev = current
            current = next
        return prev