107. Binary Tree Level Order Traversal II.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) {
  let result = [];
​
  const dfs = (root, level) => {
    if (!root) return;
​
    dfs(root.left, level + 1);
​
    if (result[level]) {
      result[level].push(root.val);
    } else {
      result[level] = [root.val];
    }
​
    dfs(root.right, level + 1);
  };
​
  dfs(root, 0);
  return result.reverse();
};
​