Introduction

In this blog post, we will explore how to solve the LeetCode problem "589" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

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] Constraints: 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?

Explanation

Here's the solution for the N-ary tree preorder traversal, addressing both recursive and iterative approaches with a focus on efficiency.

• High-Level Approach:
  • Iterative Preorder: Use a stack to simulate the recursive calls. Push the root onto the stack. While the stack is not empty, pop a node, add its value to the result, and then push its children onto the stack in reverse order to maintain the correct preorder sequence (leftmost child processed first).
  • Reverse Order Push: Pushing children in reverse order ensures that when the next node is popped from the stack, it's the leftmost child of the previously processed node.
• Complexity:
  • Runtime: O(N), where N is the number of nodes in the tree. Storage: O(H) in the average case and O(N) in the worst case(skewed tree), where H is the height of the tree.

Code

    """
# 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]:
        """
        Iterative preorder traversal of an N-ary tree.
        """
        if not root:
            return []

        result = []
        stack = [root]

        while stack:
            node = stack.pop()
            result.append(node.val)

            if node.children:
                # Push children in reverse order to maintain preorder sequence
                for child in reversed(node.children):
                    stack.append(child)

        return result