It's hard if: 1. You dont recognise the recursive nature of the problem, Or 2. You don't recognise the stack-based nature in the problem (shunting yard algo,

It's hard if: 1. You dont recognise the recursive nature of the problem, Or 2. You don't recognise the stack-based nature in the problem (shunting yard algo, etc). IOW, you did it the hard way :-).