N-ary Tree Preorder Traversal — LeetCode 589 Python Solution

The problem

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.

Example

Input

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

Output

[1,3,5,6,2,4]

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]:
        def dfs(root):
            if root is None:
                return
            ans.append(root.val)
            for child in root.children:
                dfs(child)

        ans = []
        dfs(root)
        return ans

Complexity

Pattern: Stack

When the most recent unresolved thing is the one that matters, use a stack. LeetCode 589. N-ary Tree Preorder Traversal is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Stack.

The stack guide has the Python template for the pattern and the 194 LeetCode problems that use it.

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