LeetCode: Add One Row To Tree Solution

1/**
2 * Definition for a binary tree node.
3 * function TreeNode(val, left, right) {
4 *     this.val = (val===undefined ? 0 : val)
5 *     this.left = (left===undefined ? null : left)
6 *     this.right = (right===undefined ? null : right)
7 * }
8 */
9/**
10 * @param {TreeNode} root
11 * @param {number} v
12 * @param {number} d
13 * @return {TreeNode}
14 */
15var addOneRow = function (root, v, d) {
16  if (d === 1) {
17    return new TreeNode(v, root)
18  }
19
20  const recursion = function (node, depth = 1) {
21    if (!node) {
22      return
23    }
24
25    if (depth === d - 1) {
26      node.left = new TreeNode(v, node.left, undefined)
27      node.right = new TreeNode(v, undefined, node.right)
28      return
29    }
30
31    recursion(node.left, depth + 1)
32    recursion(node.right, depth + 1)
33  }
34
35  recursion(root, 1)
36
37  return root
38}