Binary Tree Level Order Traversal

You get the root root of a binary tree. In a binary tree, each node has at most two children: a left child and a right child.

A level of the tree is the group of nodes at the same distance from the root. Return the node values level by level, as a list of lists.

Put the levels in order from the top level to the bottom level. In each level, put the values in order from left to right. For an empty tree, return an empty list.

The examples show each tree as a list in level order. This list gives the nodes row by row, from left to right. The value null marks a missing child.