LC 94. Binary Tree Inorder Traversal

 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 * @return {number[]}
12 */
13var inorderTraversal = function(root) {
14  const res = []
15  const inorder = (root) => {
16    if (!root) return
17    inorder(root.left)
18    res.push(root.val)
19    inorder(root.right)
20  }
21  inorder(root)
22  return res
23};