Binary Tree Level Order Traversal

Problem Description

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

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -1000 <= Node.val <= 1000

Solution (JavaScript)

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrder = function(root) {
    const vals = [];
    if (!root) return vals;
    
    let nodes = traverse([root], vals);
    while(nodes.length) {
        nodes = traverse(nodes, vals);
    }

    return vals;
};

function traverse(nodes, vals) {
    vals.push(nodes.map(n=>n.val));
    return nodes.map(n=>[n.left, n.right]).flat().filter(n=>!!n);
}