const levelOrder = function(root) {
 if (root == null) return []
 const result = []
 const queue = [root] 
 
 while (queue.length > 0) {
 const levelSize = queue.length
 const level = [] 
 for (let i = 0; i < levelSize; i++) { 
 const head = queue.shift()
 for (const child of head.children) {
 queue.push(child)
 }
 level.push(head.val) 
 }
 result.push(level)
 }
 return result
}