Treat individual objects and compositions of objects uniformly — a file and a folder full of files should answer getSize() the same way, recursively.
Implement `FileNode.getSize()` (returns its own size) and FolderNode.getSize()/add() (sums its children's sizes, recursively).
solve(tree) builds a tree from nested numbers/arrays (5 = a file of size 5, [a, b] = a folder containing a and b) and returns the total size.
solve([1, [2, 3], [4, [5, 6]]]) → 21
Sample tests