537. Complex Number Multiplication

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:
The input strings will not have extra blank.
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.

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public class Solution {
public String complexNumberMultiply(String a, String b) {
int[] numsa = getNums(a);
int[] numsb = getNums(b);
int n1 = numsa[0] * numsb[0] - numsa[1] * numsb[1];
int n2 = numsa[0] * numsb[1] + numsa[1] * numsb[0];
return n1 + "+" + n2 + "i";
}
private int[] getNums(String s) {
StringBuilder sb = new StringBuilder();
int i = 0;
do {
sb.append(s.charAt(i++));
} while (s.charAt(i) != '+' && s.charAt(i) != '-');
i++;
int a = Integer.parseInt(sb.toString());
sb = new StringBuilder();
while (true) {
char c = s.charAt(i);
if (c != 'i') {
sb.append(c);
i++;
} else
break;
}
int b = Integer.parseInt(sb.toString());
return new int[] { a, b };
}
}