Binary Tree Level Order Traversal

Print the tree level by level: the root on the first line, its children on the second, and so on — each level from left to right.

Tree format. A binary tree is given on one line in level order (top to bottom, left to right), with null for a missing child — the same way most interview platforms show trees. Trailing nulls are left out, and an empty tree is the single word null. Example: 3 9 20 null null 15 7 is

    3
   / \
  9   20
     /  \
    15   7

Input: one tree.

Output: one line per level, or empty for an empty tree.

Constraints: 0 ≤ nodes ≤ 10⁴. The tree has at most 10⁴ nodes and its height is at most 500, so recursion is safe in every language.

Think about it: BFS with a queue. To know where a level ends, record the queue size at the start of each level and pop exactly that many nodes.