75. implement BigInt subtraction
medium - accepted / - tried
1000000000000000000000n - 999999999999999999999n // 1n
Suppose BigInt cannot be used, can you implement a string subtraction function by yourself?
subtract('1000000000000000000000', '999999999999999999999') // '1'
All input are valid non-negative integer strings, and the result is guaranteed to be non-negative.
Don't use BigInt directly, it is not our goal here