var preorderTraversal =function(root){
const res =[]
function traverse(root){
if(root){
res.push(root.val);if(root.left){
traverse(root.left)}if(root.right){
traverse(root.right)}}return res;}traverse(root)return res;};
遍历
var preorderTraversal =function(root){
let r =[]
function fn(t){
if(t){
let stack =[t];while(stack.length){
let item = stack.shift();
r.push(item.val);if(item.right){
stack.unshift(item.right)}if(item.left){
stack.unshift(item.left)}}}}fn(root)return r;}