def my_factorial(num: int) -> int:
    """
    Solve factorial
    变量申明类型==>变量名: 变量类型
    返回值类型申明==>在参数申明和：之间， -> 变量类型

    :param num: Non-negative integer
    :return: number's factorial
    """
    result: int = 1
    for n in range(1, num + 1):
        result *= n
    return result