LeetCode-206. 反转链表

给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

代码：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        cur, pre = head, None
        while cur:
            temp = cur.next
            cur.next = pre
            pre = cur
            cur = temp
        return pre