LeetCode 589. N-ary Tree Preorder Traversal



Given 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).

Follow up:

Recursive solution is trivial, could you do it iteratively?

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 height of the n-ary tree is less than or equal to 1000
  • The total number of nodes is between [0, 10^4]


Recursively visit nodes using pre order.

Python Solution

# 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]:
        results = []
        self.helper(root, results)
        return results
    def helper(self, root, results):
        if not root:
        for child in root.children:
            self.helper(child, results)
  • Time Complexity: O(N)
  • Space Complexity: O(N)

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