LeetCode - Complex Number Multiplication

Question Definition

Given two strings representing two complex numbers.

You need to return a string representing their multiplication. Note i2 = -1 according to the definition. Example 1:

Input: "1+1i", "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: "1+-1i", "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.

Note:

1. The input strings will not have extra blank.
2. The input strings will be given in the form of a+bi, where the integer a and b will both belong to the range of [-100, 100]. And the output should be also in this form.

   Java Solution

   public String complexNumberMultiply(String a, String b) {
    int a1 = Integer.parseInt(a.substring(0, a.indexOf("+")));
    int a2 = Integer.parseInt(a.substring(a.indexOf("+") + 1, a.length() - 1));
    int b1 = Integer.parseInt(b.substring(0, b.indexOf("+")));
    int b2 = Integer.parseInt(b.substring(b.indexOf("+") + 1, b.length() - 1));
    return (a1 * b1 - a2 * b2) + "+" + (a1 * b2 + a2 * b1) + "i";
   }