Negative values are only accepted if the radix is 10, otherwise the sign is ignored. This can be tricky if it doesn't fall out of your algorithm naturally, so that's a deciding factor in how to go about doing the conversion. Ideally, if you can keep it to a boolean flag then you're doing good. :) So let's look at the stub with task comments added for the ideal solution:
If radix is 10 and value is negative the string is preceded by the minus sign (-). With any other radix, value is always considered unsigned.