/**
 * 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 inorderTraversal = function (root, list = []) {
    if (!root) return [];

    inorderTraversal(root.left, list);
    list.push(root.val);
    inorderTraversal(root.right, list);

    return list;
};