编写一个使用SQLite管理电话号码的程序，功能包括：显示所有电话号码、添加联系人、删除联系人、退出，请补全程序中空缺的①②③④部分。（本题无需运行通过，写入代码即可）

import sqlite3
def init_db():
    conn = sqlite3.connect('phonebook.db')
    cursor = conn.cursor()
    cursor.execute("DROP TABLE IF EXISTS contacts")
    cursor.execute('''CREATE TABLE contacts (name TEXT NOT NULL,phone TEXT PRIMARY KEY)''')
    conn.commit()
    return conn

def show_all_contacts(conn):
    cursor = conn.cursor()
    cursor.execute("SELECT name, phone FROM contacts ORDER BY name")
    contacts = cursor.  ①     
    if not contacts:
        print("电话簿为空")
        return
        for name, phone in contacts:
        print(f"{name}: {phone}")

def add_contact(conn):
    name = input("请输入姓名: ").strip()
    phone = input("请输入电话号码: ").strip()

    if not name or not phone:
        print("错误: 姓名和电话都不能为空")
        return

    try:
        cursor = conn.cursor()
        cursor.execute("INSERT INTO contacts (name, phone) VALUES (?, ?)", ( ② , ③ ))
        conn.commit()
        print(f"成功添加联系人: {name} - {phone}")
    except sqlite3.IntegrityError:
        print("错误: 该电话号码已存在")

def delete_contact(conn):
    phone = input("请输入要删除的电话号码: ").strip()

    if not phone:
        print("错误: 请输入有效的电话号码")
        return

    cursor = conn.cursor()
    cursor.execute("SELECT name FROM contacts WHERE phone=?", (phone,))
    result = cursor.   ④      
    if not result:
        print("错误: 未找到该电话号码")
        return

    confirm = input(f"确定要删除 {result[0]}({phone}) 吗? (y/n): ").lower()
    if confirm == 'y':
        cursor.execute("DELETE FROM contacts WHERE phone=?", (phone,))
        conn.commit()
        print("联系人已删除")
    else:
        print("取消删除")

# 主程序
def main():
    conn = init_db()

    while True:
        print("----------------")
        show_all_contacts(conn)
        print("\n请选择操作:")
        print("1. 添加联系人")
        print("2. 删除联系人")
        print("0. 退出程序")

        choice = input("请输入选项(0-2): ").strip()

        if choice == "1":
            add_contact(conn)
        elif choice == "2":
            delete_contact(conn)
        elif choice == "0":
            print("\n感谢使用电话簿管理系统，再见!")
            conn.close()
            break
        else:
            print("无效输入，请重新选择!")

if __name__ == "__main__":
    main()