Performancewise would be "better" to use a StringBuilder or StringBuffer (using the .append() method) as it is faster to reallocate memory for a growing object, than creating a new object in each loop iteration.

I like this solution best, because it works when n and m are negative (although the problem statement claims the inputs will never be negative).

Not true. If n or m are 0, first if will be true, and return 0

I think that's the best and readable solution :)