text: /**
   * 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 preorderTraversal = function(root, array = []) {
    if (root === null) {
      return;
    }
    array.push(root.val);
    preorderTraversal(root.left, array);
    preorderTraversal(root.right, array);
    return array;
  };