题解 | 反转链表

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @return ListNode类
#
class Solution:
    def ReverseList(self , head: ListNode) -> ListNode:
        # write code here
        if not head:  #如果链表为空
            return None
        prev = None
        curr = head
        while curr:
            next_node = curr.next  #记录下一个节点
            curr.next = prev  #反转指向
            prev = curr  #把prev向前移动
            curr = next_node  #把curr向前移动
        return prev