A complex number can be represented as a string on the form `"real+imaginaryi"` where:

- `real` is the real part and is an integer in the range `[-100, 100]`.
- `imaginary` is the imaginary part and is an integer in the range `[-100, 100]`.
- `i2 == -1`.

Given two complex numbers `num1` and `num2` as strings, return a string of the complex number that represents their multiplications.

Example 1:

    Input: num1 = "1+1i", num2 = "1+1i"
    Output: "0+2i"
    Explanation: (1 + i) * (1 + i) = 1 + i2 + 2 * i = 2i, and you need convert it to the form of 0+2i.

Example 2:

    Input: num1 = "1+-1i", num2 = "1+-1i"
    Output: "0+-2i"
    Explanation: (1 - i) * (1 - i) = 1 + i2 - 2 * i = -2i, and you need convert it to the form of 0+-2i.

Constraints:

- `num1` and `num2` are valid complex numbers.