单链表声明节点，初始化头节点，建立头插和尾插，按值删找写一个代码里并覆值操作两次

在Python中，我们可以使用一个Node类表示单链表的节点，然后定义一个LinkedList类来处理链表的操作。这里是一个简单的示例，包括节点的声明、初始化头节点、头部插入、尾部插入以及按值删除和覆盖操作：

class Node:
    def __init__(self, data=None):
        self.data = data
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None

    # 插入头节点
    def insert_at_head(self, data):
        new_node = Node(data)
        if self.head is not None:
            new_node.next = self.head
        self.head = new_node

    # 尾部插入
    def insert_at_tail(self, data):
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
            return
        last_node = self.head
        while last_node.next:
            last_node = last_node.next
        last_node.next = new_node

    # 按值查找并删除节点（如果存在）
    def delete_by_value(self, value):
        current = self.head
        if current and current.data == value:
            self.head = current.next
            current = None
            return
        prev = None
        while current and current.data != value:
            prev = current
            current = current.next
        if current:
            prev.next = current.next
            current = None

    # 覆盖操作（例如替换最后一个节点的值）
    def override_last_value(self, new_data):
        if self.head is not None:
            last_node = self.head
            while last_node.next:
                last_node = last_node.next
            last_node.data = new_data

# 示例
ll = LinkedList()
ll.insert_at_head(1)  # [1]
ll.insert_at_head(0)  # [0, 1]
ll.insert_at_tail(2)   # [0, 1, 2]
print("Before deletion:", ll)  # [0, 1, 2]

ll.delete_by_value(1)  # [0, 2]
print("After deleting 1:", ll)  # [0, 2]

ll.override_last_value(3)  # [0, 2] -> [0, 3]
print("After overriding last value:", ll)  # [0, 3]