给定一个 N 叉树，返回其节点值的前序遍历。
Iterative: class Solution(object): def preorder(self, root): """ :type root: Node :rtype: List[int] """ if not root: return [] res= [] stack = [root] while stack: cur = stack.pop() res.append(cur.val) stack.extend(reversed(cur.children)) return res Recursive: class Solution(object): def preorder(self, root): """ :type root: Node :rtype: List[int] """ if not root: return [] res= [root.val] for child in root.children: res.extend(self.preorder(child)) return res