const preorder = function(root) {
 if (root === null) return []
 const res = []
 function dfs(root) {
 if (root === null) return
 res.push(root.val)
 for (let i = 0; i < root.children.length; i++) {
 dfs(root.children[i])
 }
 }
 dfs(root)
 return res
}