Its a problem that's trivial if you are allowed to allocate an array/list that's the same size as the input but tricky if you have to do the operation in place

Its a problem that's trivial if you are allowed to allocate an array/list that's the same size as the input but tricky if you have to do the operation in place (unless you already know the fisher yates algorithm). There's probably a bunch of other examples of this kind, (easy out of place, tricky in place) but non come to mind atm.