设计一个复数类，能够完成复数之间的基本运算，重写方法toString()，使其能输出此复数（形式为： 实部 + 虚部i）。

要求设计的复数类必须有三个构造函数，分别为无参数、1个参数和2个参数的构造函数，完成的基本运算包括两个复数的加、减、乘、除法和变换了共轭复数。

-The design of a complex type, able to complete the basic operations between plural, rewrite the method toString (), so that it can output the plural (in the form as follows: real part+ imaginary part i). Designed to be the plural there are three types of structural function, respectively, without parameters, a parameter and two constructor parameters, including the completion of the basic operations of the two complex add, subtract, multiply, divide and transform the complex conjugate.