BinaryTreeLevelOrderTraversal.js

// Source : https://oj.leetcode.com/problems/binary-tree-level-order-traversal/
// Author : Dean Shi
// Date   : 2015-05-29

/**********************************************************************************
 *
 * 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,#,#,15,7},
 *
 *     3
 *    / \
 *   9  20
 *     /  \
 *    15   7
 *
 * return its level order traversal as:
 *
 * [
 *   [3],
 *   [9,20],
 *   [15,7]
 * ]
 *
 * confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.
 *
 * OJ's Binary Tree Serialization:
 *
 * The serialization of a binary tree follows a level order traversal, where '#' signifies
 * a path terminator where no node exists below.
 *
 * Here's an example:
 *
 *    1
 *   / \
 *  2   3
 *     /
 *    4
 *     \
 *      5
 *
 * The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".
 *
 *
 **********************************************************************************/

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */

/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function(root, index = 0, arr = []) {
    if (root) {
        if (!arr[index]) arr[index] = []
        arr[index].push(root.val)
        levelOrder(root.left, index + 1, arr)
        levelOrder(root.right, index + 1, arr)    
    }
    
    return arr
};