Find Root of N-Ary Tree

In this problem, you are given all the nodes of an N-ary tree as an array of Node objects, and you need to find and return the root of the N-ary tree.

Since the root of the tree is the only node that is not a child of any other node, we can find the root by following these steps:

  1. Create a Set of Child Nodes: Iterate through all the nodes in the tree, and add all the child nodes to a set.
  2. Find the Root: Iterate through all the nodes in the tree again, and look for a node that is not in the set of child nodes. That node will be the root.

Here’s a Python code to achieve this:

class Solution:
    def findRoot(self, tree: List['Node']) -> 'Node':
        # Create a set to store all child nodes
        child_nodes = set()
        for node in tree:
            for child in node.children:

        # Find a node that is not in the set of child nodes (i.e., the root)
        for node in tree:
            if node not in child_nodes:
                return node


  • The child_nodes set will contain all the nodes that are children of some other nodes.
  • When we iterate through the tree and find a node that is not in the child_nodes set, that node must be the root, as it is not a child of any other node.

This code has a time complexity of (O(n)), where (n) is the total number of nodes in the tree, and it correctly finds and returns the root of the N-ary tree as per the problem’s requirements.

Identifying Problem Isomorphism

“Find Root of N-Ary Tree” can be mapped to “Find Duplicate Subtrees”.

In “Find Root of N-Ary Tree”, you need to identify the root from a set of nodes given in an arbitrary order. The root is the node that no other node points to, so it’s a unique element in the context of the tree.

In “Find Duplicate Subtrees”, you need to identify all the duplicate subtrees from a binary tree. Even though the tasks are different, the approach for solving both problems involves tree traversal and comparison of tree nodes.

“Find Root of N-Ary Tree” is simpler because it involves finding a single node in an array of nodes. “Find Duplicate Subtrees” is more complex because it involves identifying duplicate subtrees in a binary tree, which requires traversal and serialization of subtrees.

These two problems share a fundamental principle of tree traversal and node comparison. Hence, they are approximately isomorphic.

10 Prerequisite LeetCode Problems

For “1506. Find Root of N-Ary Tree”, the following is a good preparation:

  1. “429. N-ary Tree Level Order Traversal”: This problem will help understand traversing N-ary tree in a level order fashion which is key to understanding the structure of the tree in the main problem.

  2. “590. N-ary Tree Postorder Traversal”: This problem will teach how to traverse an N-ary tree in a postorder manner. Understanding various tree traversals is helpful for the main problem.

  3. “559. Maximum Depth of N-ary Tree”: This problem helps you understand the concept of depth in an N-ary tree, which is essential to identify the root node.

  4. “589. N-ary Tree Preorder Traversal”: The problem is another variant of N-ary tree traversal, this time in a preorder manner. This will help understand the tree structure, which is key to solving the main problem.

  5. “700. Search in a Binary Search Tree”: Although this problem is about binary trees, the search operation is common to all tree data structures and will help in understanding how to locate a specific node.

  6. “104. Maximum Depth of Binary Tree”: Understanding the concept of tree depth in a simpler context of binary trees can be helpful before moving onto the main problem.

  7. “101. Symmetric Tree”: This problem helps in understanding tree structures better, which is essential for locating the root node in the main problem.

  8. “226. Invert Binary Tree”: This problem is about modifying a binary tree, which enhances the overall understanding of tree manipulation.

  9. “617. Merge Two Binary Trees”: It deals with manipulation of two binary trees simultaneously, further enhancing tree manipulation understanding.

  10. “102. Binary Tree Level Order Traversal”: This problem teaches how to perform a level order traversal in a binary tree, providing foundational knowledge for handling the N-ary tree in the main problem.

These cover tree structures, tree traversals, and handling of N-ary trees, which are necessary for tackling the main problem.

Problem Classification

Problem Statement:You are given all the nodes of an N-ary tree as an array of Node objects, where each node has a unique value. Return the root of the N-ary tree. Custom testing: An N-ary tree can be serialized as represented in its level order traversal where each group of children is separated by the null value (see examples). For example, the above tree is serialized as [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].

The testing will be done in the following way:

The input data should be provided as a serialization of the tree. The driver code will construct the tree from the serialized input data and put each Node object into an array in an arbitrary order. The driver code will pass the array to findRoot, and your function should find and return the root Node object in the array. The driver code will take the returned Node object and serialize it. If the serialized value and the input data are the same, the test passes.

Example 1:

Input: tree = [1,null,3,2,4,null,5,6] Output: [1,null,3,2,4,null,5,6] Explanation: The tree from the input data is shown above. The driver code creates the tree and gives findRoot the Node objects in an arbitrary order. For example, the passed array could be [Node(5),Node(4),Node(3),Node(6),Node(2),Node(1)] or [Node(2),Node(6),Node(1),Node(3),Node(5),Node(4)]. The findRoot function should return the root Node(1), and the driver code will serialize it and compare with the input data. The input data and serialized Node(1) are the same, so the test passes.

Example 2:

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


The total number of nodes is between [1, 5 * 104]. Each node has a unique value.

Follow up:

Could you solve this problem in constant space complexity with a linear time algorithm?

Analyze the provided problem statement. Categorize it based on its domain, ignoring ‘How’ it might be solved. Identify and list out the ‘What’ components. Based on these, further classify the problem. Explain your categorizations.

Clarification Questions

What are the clarification questions we can ask about this problem?

Problem Analysis and Key Insights

What are the key insights from analyzing the problem statement?

Problem Boundary

What is the scope of this problem?

How to establish the boundary of this problem?

Distilling the Problem to Its Core Elements

Can you identify the fundamental concept or principle this problem is based upon? Please explain. What is the simplest way you would describe this problem to someone unfamiliar with the subject? What is the core problem we are trying to solve? Can we simplify the problem statement? Can you break down the problem into its key components? What is the minimal set of operations we need to perform to solve this problem?

Visual Model of the Problem

How to visualize the problem statement for this problem?

Problem Restatement

Could you start by paraphrasing the problem statement in your own words? Try to distill the problem into its essential elements and make sure to clarify the requirements and constraints. This exercise should aid in understanding the problem better and aligning our thought process before jumping into solving it.

Abstract Representation of the Problem

Could you help me formulate an abstract representation of this problem?

Given this problem, how can we describe it in an abstract way that emphasizes the structure and key elements, without the specific real-world details?


Are there any specialized terms, jargon, or technical concepts that are crucial to understanding this problem or solution? Could you define them and explain their role within the context of this problem?

Problem Simplification and Explanation

Could you please break down this problem into simpler terms? What are the key concepts involved and how do they interact? Can you also provide a metaphor or analogy to help me understand the problem better?


Given the problem statement and the constraints provided, identify specific characteristics or conditions that can be exploited to our advantage in finding an efficient solution. Look for patterns or specific numerical ranges that could be useful in manipulating or interpreting the data.

What are the key insights from analyzing the constraints?

Case Analysis

Could you please provide additional examples or test cases that cover a wider range of the input space, including edge and boundary conditions? In doing so, could you also analyze each example to highlight different aspects of the problem, key constraints and potential pitfalls, as well as the reasoning behind the expected output for each case? This should help in generating key insights about the problem and ensuring the solution is robust and handles all possible scenarios.

Provide names by categorizing these cases

What are the edge cases?

How to visualize these cases?

What are the key insights from analyzing the different cases?

Identification of Applicable Theoretical Concepts

Can you identify any mathematical or algorithmic concepts or properties that can be applied to simplify the problem or make it more manageable? Think about the nature of the operations or manipulations required by the problem statement. Are there existing theories, metrics, or methodologies in mathematics, computer science, or related fields that can be applied to calculate, measure, or perform these operations more effectively or efficiently?

Simple Explanation

Can you explain this problem in simple terms or like you would explain to a non-technical person? Imagine you’re explaining this problem to someone without a background in programming. How would you describe it? If you had to explain this problem to a child or someone who doesn’t know anything about coding, how would you do it? In layman’s terms, how would you explain the concept of this problem? Could you provide a metaphor or everyday example to explain the idea of this problem?

Problem Breakdown and Solution Methodology

Given the problem statement, can you explain in detail how you would approach solving it? Please break down the process into smaller steps, illustrating how each step contributes to the overall solution. If applicable, consider using metaphors, analogies, or visual representations to make your explanation more intuitive. After explaining the process, can you also discuss how specific operations or changes in the problem’s parameters would affect the solution? Lastly, demonstrate the workings of your approach using one or more example cases.

Inference of Problem-Solving Approach from the Problem Statement

Can you identify the key terms or concepts in this problem and explain how they inform your approach to solving it? Please list each keyword and how it guides you towards using a specific strategy or method. How can I recognize these properties by drawing tables or diagrams?

How did you infer from the problem statement that this problem can be solved using ?

Simple Explanation of the Proof

I’m having trouble understanding the proof of this algorithm. Could you explain it in a way that’s easy to understand?

Stepwise Refinement

  1. Could you please provide a stepwise refinement of our approach to solving this problem?

  2. How can we take the high-level solution approach and distill it into more granular, actionable steps?

  3. Could you identify any parts of the problem that can be solved independently?

  4. Are there any repeatable patterns within our solution?

Solution Approach and Analysis

Given the problem statement, can you explain in detail how you would approach solving it? Please break down the process into smaller steps, illustrating how each step contributes to the overall solution. If applicable, consider using metaphors, analogies, or visual representations to make your explanation more intuitive. After explaining the process, can you also discuss how specific operations or changes in the problem’s parameters would affect the solution? Lastly, demonstrate the workings of your approach using one or more example cases.

Identify Invariant

What is the invariant in this problem?

Identify Loop Invariant

What is the loop invariant in this problem?

Is invariant and loop invariant the same for this problem?

Thought Process

Can you explain the basic thought process and steps involved in solving this type of problem?

Explain the thought process by thinking step by step to solve this problem from the problem statement and code the final solution. Write code in Python3. What are the cues in the problem statement? What direction does it suggest in the approach to the problem? Generate insights about the problem statement.

Establishing Preconditions and Postconditions

  1. Parameters:

    • What are the inputs to the method?
    • What types are these parameters?
    • What do these parameters represent in the context of the problem?
  2. Preconditions:

    • Before this method is called, what must be true about the state of the program or the values of the parameters?
    • Are there any constraints on the input parameters?
    • Is there a specific state that the program or some part of it must be in?
  3. Method Functionality:

    • What is this method expected to do?
    • How does it interact with the inputs and the current state of the program?
  4. Postconditions:

    • After the method has been called and has returned, what is now true about the state of the program or the values of the parameters?
    • What does the return value represent or indicate?
    • What side effects, if any, does the method have?
  5. Error Handling:

    • How does the method respond if the preconditions are not met?
    • Does it throw an exception, return a special value, or do something else?

Problem Decomposition

  1. Problem Understanding:

    • Can you explain the problem in your own words? What are the key components and requirements?
  2. Initial Breakdown:

    • Start by identifying the major parts or stages of the problem. How can you break the problem into several broad subproblems?
  3. Subproblem Refinement:

    • For each subproblem identified, ask yourself if it can be further broken down. What are the smaller tasks that need to be done to solve each subproblem?
  4. Task Identification:

    • Within these smaller tasks, are there any that are repeated or very similar? Could these be generalized into a single, reusable task?
  5. Task Abstraction:

    • For each task you’ve identified, is it abstracted enough to be clear and reusable, but still makes sense in the context of the problem?
  6. Method Naming:

    • Can you give each task a simple, descriptive name that makes its purpose clear?
  7. Subproblem Interactions:

    • How do these subproblems or tasks interact with each other? In what order do they need to be performed? Are there any dependencies?

From Brute Force to Optimal Solution

Could you please begin by illustrating a brute force solution for this problem? After detailing and discussing the inefficiencies of the brute force approach, could you then guide us through the process of optimizing this solution? Please explain each step towards optimization, discussing the reasoning behind each decision made, and how it improves upon the previous solution. Also, could you show how these optimizations impact the time and space complexity of our solution?

Code Explanation and Design Decisions

  1. Identify the initial parameters and explain their significance in the context of the problem statement or the solution domain.

  2. Discuss the primary loop or iteration over the input data. What does each iteration represent in terms of the problem you’re trying to solve? How does the iteration advance or contribute to the solution?

  3. If there are conditions or branches within the loop, what do these conditions signify? Explain the logical reasoning behind the branching in the context of the problem’s constraints or requirements.

  4. If there are updates or modifications to parameters within the loop, clarify why these changes are necessary. How do these modifications reflect changes in the state of the solution or the constraints of the problem?

  5. Describe any invariant that’s maintained throughout the code, and explain how it helps meet the problem’s constraints or objectives.

  6. Discuss the significance of the final output in relation to the problem statement or solution domain. What does it represent and how does it satisfy the problem’s requirements?

Remember, the focus here is not to explain what the code does on a syntactic level, but to communicate the intent and rationale behind the code in the context of the problem being solved.

Coding Constructs

Consider the code for the solution of this problem.

  1. What are the high-level problem-solving strategies or techniques being used by this code?

  2. If you had to explain the purpose of this code to a non-programmer, what would you say?

  3. Can you identify the logical elements or constructs used in this code, independent of any programming language?

  4. Could you describe the algorithmic approach used by this code in plain English?

  5. What are the key steps or operations this code is performing on the input data, and why?

  6. Can you identify the algorithmic patterns or strategies used by this code, irrespective of the specific programming language syntax?

Language Agnostic Coding Drills

Your mission is to deconstruct this code into the smallest possible learning units, each corresponding to a separate coding concept. Consider these concepts as unique coding drills that can be individually implemented and later assembled into the final solution.

  1. Dissect the code and identify each distinct concept it contains. Remember, this process should be language-agnostic and generally applicable to most modern programming languages.

  2. Once you’ve identified these coding concepts or drills, list them out in order of increasing difficulty. Provide a brief description of each concept and why it is classified at its particular difficulty level.

  3. Next, describe the problem-solving approach that would lead from the problem statement to the final solution. Think about how each of these coding drills contributes to the overall solution. Elucidate the step-by-step process involved in using these drills to solve the problem. Please refrain from writing any actual code; we’re focusing on understanding the process and strategy.

Targeted Drills in Python

Now that you’ve identified and ordered the coding concepts from a complex software code in the previous exercise, let’s focus on creating Python-based coding drills for each of those concepts.

  1. Begin by writing a separate piece of Python code that encapsulates each identified concept. These individual drills should illustrate how to implement each concept in Python. Please ensure that these are suitable even for those with a basic understanding of Python.

  2. In addition to the general concepts, identify and write coding drills for any problem-specific concepts that might be needed to create a solution. Describe why these drills are essential for our problem.

  3. Once all drills have been coded, describe how these pieces can be integrated together in the right order to solve the initial problem. Each drill should contribute to building up to the final solution.

Remember, the goal is to not only to write these drills but also to ensure that they can be cohesively assembled into one comprehensive solution.


Similar Problems

Can you suggest 10 problems from LeetCode that require similar problem-solving strategies or use similar underlying concepts as the problem we’ve just solved? These problems can be from any domain or topic, but they should involve similar steps or techniques in the solution process. Also, please briefly explain why you consider each of these problems to be related to our original problem. The response text is of the following format:

Here are 10 problems that use similar underlying concepts: