Apart from theorems, their proofs, and certain derivations, there isn’t much of a restriction as to the route the student must take to reach the final solution. Various third-party books offer different ways to solve the same problem, and students are free to use the method they are most comfortable with unless a particular method is explicitly mentioned in the question. The step-marking criteria are mostly taken into consideration when a particular method is mentioned in the question. If it is not, then whatever method students use is accepted and also, marked justly.