设计一个单链表的基本运算程序，并用相关数据进行测试

设计一个单链表的基本运算程序通常涉及以下几个步骤：

1. **定义链表结构**：首先创建一个节点类，包含一个数据字段和一个指向下一个节点的引用（指针）。

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

2. **链表类**：定义链表类，包括初始化、插入节点、删除节点、搜索和打印列表等基本操作。

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

    def append(self, data):
        if not self.head:
            self.head = Node(data)
        else:
            current = self.head
            while current.next:
                current = current.next
            current.next = Node(data)

    def insert_at(self, index, data):
        if index < 0 or index > self.length():
            return
        new_node = Node(data)
        if index == 0:
            new_node.next = self.head
            self.head = new_node
        else:
            current = self.head
            for _ in range(index - 1):
                current = current.next
            new_node.next = current.next
            current.next = new_node

    def delete(self, key):
        if not self.head:
            return
        if self.head.data == key:
            self.head = self.head.next
            return
        current = self.head
        while current.next and current.next.data != key:
            current = current.next
        if current.next:
            current.next = current.next.next

    def search(self, key):
        current = self.head
        while current:
            if current.data == key:
                return True
            current = current.next
        return False

    def print_list(self):
        elements = []
        current = self.head
        while current:
            elements.append(current.data)
            current = current.next
        print("List:", elements)
    
    def length(self):
        count = 0
        current = self.head
        while current:
            count += 1
            current = current.next
        return count

你可以通过以下方式进行测试：

# 创建一个链表实例
my_list = LinkedList()

# 添加元素
my_list.append(1)
my_list.append(2)
my_list.insert_at(1, 3)  # 插入 3 在索引 1

# 打印初始链表
my_list.print_list()  # 输出: List: [1, 3, 2]

# 搜索和删除
print(my_list.search(3))  # 输出: True
my_list.delete(2)
my_list.print_list()  # 输出: List: [1, 3]

# 验证长度
print(my_list.length())  # 输出: 2