var levelOrder = function(root) {
if (root ==null){
return []
}
let res = []
let resItem = []
let stack2 =[root]
let stack1 = []
while(stack1.length!==0||stack2.length!==0){
while(stack2.length){
let node = stack2.pop()
resItem.push(node.val)
if (node.left){
stack1.push(node.left)
}
if (node.right){
stack1.push(node.right)
}
}
res.push(resItem)
resItem=[]
while(stack1.length){
let node = stack1.pop()
resItem.push(node.val)
if (node.right){
stack2.push(node.right)
}
if (node.left){
stack2.push(node.left)
}
}
if (resItem.length){
res.push(resItem)
}
resItem=[]
}
return res
};