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.

其实可以用一次遍历的方法求 但是我觉得这样的代码复用度不高，所以写了取不同part的函数，代价是牺牲一定的时间效率取得更高的扩展性。

class Solution {
    public String complexNumberMultiply(String a, String b) {
        StringBuilder sb = new StringBuilder();
        int real1 = get_part(a,true);
        int real2 = get_part(b,true);
        int imag1=get_part(a,false);
        int imag2=get_part(b,false);
        int real = real1*real2-imag1*imag2;
        int imag = real1*imag2+real2*imag1;
        sb.append(real);
        sb.append('+');
        sb.append(imag);
        sb.append('i');
        return sb.toString();
    }
    private int get_part(String a ,boolean real)
    {
        StringBuilder sb = new StringBuilder();
        int pos = 0;
        for(int i=0;i