function helper(root, nodes) {
if (!root) return;
helper(root.left, nodes);
nodes.push(root);
helper(root.right, nodes);
}
/**
* @param {TreeNode} root
* @param {number} k
* @return {TreeNode}
*/
var kthNode = function(root, k) {
const nodes = [];
helper(root, nodes);
return nodes[k - 1];
};