Tree BFS

function levelOrder(root: TreeNode | null): number[][] {
  const res: number[][] = [];
  if (!root) return res;
  let level: TreeNode[] = [root];
  while (level.length) {
    const vals: number[] = [];
    const next: TreeNode[] = [];
    for (const node of level) {
      vals.push(node.val);
      if (node.left) next.push(node.left);
      if (node.right) next.push(node.right);
    }
    res.push(vals);
    level = next;
  }
  return res;
}

TreeNode = Struct.new(:val, :left, :right)

def level_order(root)
  return [] unless root

  res = []
  level = [root]
  until level.empty?
    res << level.map(&:val)
    level = level.flat_map { |node| [node.left, node.right].compact }
  end
  res
end