/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function(root) {
    var result = [];
    if (root == null) {
        return result;
    }
    return result.concat(inorderTraversal(root.left), root.val, inorderTraversal(root.right));
};