This is the scenario:
1. user inputs an int N for the desired level
2. if N is greater than the total number of levels in the tree, repeat input
3. if N is less than 0, repeat input
4. if N is okay, print all the nodes belonging to this level.
1-3 I know how to do, but 4 is a mystery
I thought about putting all the nodes into an array, and then manually checking each of them if they are of the required level, but I'm not sure if that would take for ever