114. implement BigInt multiplication

medium  - accepted / - tried

This is a follow-up on 76. implement BigInt addition with sign.

You are asked to create a function that multiplies two big integers in string.

multiply(  '1123456787654323456789',   '1234567887654323456')// '1386983673205309924427166592431045142784'

What is time & space complexity of your approach?

(16)