levelOrder = function(root, level = 0, levels = []) {
    const isBaseCase = root === null;
    if (isBaseCase) return levels;

    const isLastNode = level === levels.length;
    if (isLastNode) levels.push([]);

    levels[level].push(root.val);

    return dfs(root, level, levels); // Time O(N) | Space O(H)
}
https://leetcode.com/problems/binary-tree-level-order-traversal/
Time O(N) | Space O(H)
@param {TreeNode} root
@return {number[]}