https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

levelOrderTraversalII.js

/**
 * 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 levelOrderBottom = function(root) {
    const result = [];
    
    const pot = (node, depth) => {
        if(!node) return;
        
        if(!result[depth]) result[depth] = [];
        result[depth].push(node.val);
        
        pot(node.left, depth+1);
        pot(node.right, depth+1);
    };
    
    pot(root, 0);
    
    return result.reverse();
};