Binary Tree Level Order Traversal

Description

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

Solution(javascript)

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
const levelOrder = (root) => {
  const aux = (node, level, acc) => {
    if (!node) {
      return acc
    }
    if (node.left) {
      aux(node.left, level + 1, acc)
    }
    if (acc[level]) {
      acc[level].push(node.val)
    } else {
      acc[level] = [node.val]
    }
    if (node.right) {
      aux(node.right, level + 1, acc)
    }
    return acc
  }
  return aux(root, 0, [])
}