Questions about designing algorithms, their correctness, or their complexity fit Computer Science best. This is likely to be the best fit for questions that you may have that arise in an algorithms course that is part of a computer science educational program.
If you implement algorithms as part of the course, then questions about the coding part should be asked on Stack OverflowStack Overflow, e.g. “why doesn't my code work?” (post your whole codeyour whole code and explain the desired behavior), what library functions to use, etc.
If your implementation is working and you want to make it better, take it to Code Review.
Questions about algorithms in the context of analyzing and designing software systems are a good fit on Software Engineering.
Questions about numerical algorithms in the context of applications to other sciences (statistics, physics, biology, etc.) may be a better fit for Computational Science.
Research-level questions may also be suitable on Theoretical Computer Science.