537. 复数乘法

class Solution:
    def complexNumberMultiply(self, num1: str, num2: str) -> str:
        def get_real_i(nums:str):
            #give str "a+bi" return a,b            
            i = nums.index('+')
            a = int(nums[:i])
            b = int(nums[i+1:-1])
            return a,b 
 
        a1,b1 =get_real_i(num1)
        a2,b2 =get_real_i(num2)
        a = a1*a2 - b1*b2
        b = a1*b2 + a2*b1 
        return f"{a}+{b}i"