Build the PrimitiveMedium
Binary Tree Level Order Traversal
Write `level_order(root)` returning a list of lists, one per level, top to bottom and left to right.
[3,9,20,null,null,15,7] -> [[3], [9, 20], [15, 7]]
A plain BFS visits nodes in the right order but loses the level boundaries. Capture `len(queue)` before the inner loop and drain exactly that many — the queue grows while you work, so reading the length inside the loop merges the levels.
What to expect: A timer starts when you begin. Edit the starter code, run it against the test suite as many times as you like, then finish when you're done. The reference solution and interviewer follow-up questions unlock only after you finish.