Actually with the requirements specified as returning a string, you don't really need three test cases. Just put the i%3 test case before the i%5 test case and fizz then buzz will be added to the string resulting in fizzbuzz for the divisible by three and five test case. Use a flag to specify if either case gets triggered and if not then add the number to the string.

Why not let output be a string, then just build by concatenating with += after evaluating i? Could also demonstrate type coercion when i (integer) is coerced to string.

one of the test failing with result - secondTest ✘ expected: but was:

The expected string is strange, it doesn't have any numbers. Clearly the expected result is wrong.

[deleted]

My code is as follows
public static String fizzBuzz(Integer num) {
StringBuffer result = new StringBuffer();
for (int n=1; n<=num; n++) {
if (num % 3 == 0 && num % 5 == 0) result.append("fizzbuzz");
else if (num % 3 == 0) result.append("fizz");
else if (num % 5 == 0) result.append("buzz");
else result.append(String.valueOf(n));
}
return result.toString();
}

My bad. All tests pass now. Earlier, in the failed test case the expected string was a bit strange and the real issue got overlooked. Thanks.

"Expected space complexity: O(1)"

That seems hard to achieve, won't the resulting string itself always take up O(n) space?

Your suggested python solution is O(n) space complexity and the expected space complexity is O(1) . How can you claim it as final solution ? either change constraint on space complexity to O(n) or provide accurate solution..