77. implement BigInt subtraction with sign

medium  - accepted / - tried

This is a follow-up on 75. implement BigInt subtraction.

You are asked to implement a string substraction function, with possible negative integers. Also, '+' plus sign should also be supported

substract('-999999999999999999', '-1')// '-999999999999999998'substract('-999999999999999999', '+1')// '-1000000000000000000'

Don't use BigInt directly, it is not our goal here.

Think about the edge cases.

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