Binary Tree Level Order Traversal

Given the root of a binary tree, return the level-order traversal of its node values: one array per depth level, from the root level downward, with each level's values listed left to right.

The empty tree yields an empty result.

Tree Encoding

Your function receives root as a TreeNode (or as null for the empty tree). The node type is provided for you, with val, left, and right fields.

The examples below write the tree as its level-order traversal, where null marks a missing child of a listed node and trailing nulls are omitted. That is only how the input is displayed; the decoding is done for you.