[algorithm][leetcode] 102. Binary Tree Level Order Traversal

/**
 * 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) {
    
    const arr = [];
    const flat = (node, height) => {
        if (!node) return;
        arr[height] = arr[height] || [];
        
        arr[height].push(node.val);
        height++;
        flat(node.left, height);
        flat(node.right, height);
    }
    
    flat(root, 0);
    // console.log(arr);
    return arr;
};