589. N-ary Tree Preorder Traversal

589. N-ary Tree Preorder Traversal
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Given the root of an n-ary tree, return the preorder traversal of its nodes' values.

Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

Example 1:

Input: root = [1,null,3,2,4,null,5,6] Output: [1,3,5,6,2,4]

Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14] Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]


  • The number of nodes in the tree is in the range [0, 104].
  • 0 <= Node.val <= 104
  • The height of the n-ary tree is less than or equal to 1000.

Follow up: Recursive solution is trivial, could you do it iteratively?


# Definition for a Node.
class Node:
    def __init__(self, val=None, children=None):
        self.val = val
        self.children = children

class Solution:
    def preorder(self, root: 'Node') -> List[int]:
        # pre order means we travel all the way down the left before we go to the next node over
        # so it's a DFS
        self.output = []
        def dfs(root):
            # basecase
            if root is None:
            for child in root.children:
        return self.output