I don't think giving change equal to zero should be included in the test case. Rather than giving an answer, such edge case should be kept undefined here. The instruction has mentioned nothing about zero change so we cannot infer a specific definition to such condition. "Don't give any coins" is an ambiguous definition here.　For every other solutions, you can always specifiy a list of numbers that corresponds to the solution. But you cannot write down anythin for dont' give any coins. The instructions should mention this explicitly. For now it sounds like that I'm asking how can I eat a BigMac and am told that just don't eat.