原题链接
递归 dfs
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
}