Leetcode537. Complex multiplication (yes, solved)

The plural Can be expressed as a string , follow “ Real component + Imaginary part i” In the form of , And meet the following conditions ：

• Real component It's an integer , The value range is [-100, 100]
• Imaginary part It's also an integer , The value range is [-100, 100]
• i2 == -1

Give you a complex number represented by two strings num1 and num2 , Please follow the plural form , Returns a string representing their product .

Example 1：

 Input ：num1 = "1+1i", num2 = "1+1i"
 Output ："0+2i"
 explain ：(1 + i) * (