One thing is: you have 30 minutes of time and can compile and test and end with a working implementation. Another thing is write the code on a whiteboard, that

One thing is: you have 30 minutes of time and can compile and test and end with a working implementation. Another thing is write the code on a whiteboard, that is a process nobody ever follows in practice: very easy to make mistakes this way. Anyway doubly linked lists are easy to get wrong without compiling / testing.