链表反转

class ListNode:
    def __init__(self, value=0, next=None):
        self.value = value
        self.next = next

def reverse_list(head):
    prev = None
    current = head
    while current:
        next_node = current.next  # 暂存下一个节点
        current.next = prev       # 反转当前节点的指针
        prev = current            # 移动prev和current指针
        current = next_node
    return prev  # prev现在是新的头节点

# 创建链表 1 -> 2 -> 3 -> None
head = ListNode(1, ListNode(2, ListNode(3)))

# 反转链表
new_head = reverse_list(head)

# 打印反转后的链表
current = new_head
while current:
    print(current.value, end=" -> ")
    current = current.next
# 输出: 3 -> 2 -> 1 -> None