Java Coding Test Course, Finding the Parent of a Tree

Coding tests are a mandatory requirement for many companies, and understanding data structures and algorithms has become a core competency. In this article, we will address the tree structure and the problem of finding its parent node. We will detail the importance of this problem, the methods to solve it, and the process of solving it in Java.

Problem Description

A tree structure is a data structure that represents the hierarchical relationship between nodes. Each node can have child nodes, and within this simple structure, various problems can arise. The problem at hand is to find the parent node of a given node.

Problem Definition

Write a program that takes the given tree and the value of a specific node as input and returns the corresponding parent node. If the input node is the root node or does not exist, it should return -1.

Input Format

First line: number of nodes  (1 ≤ N ≤ 100,000)
Second line: parent node information of each node (represented as space-separated integers)
Third line: the node to be found

Output Format

Value of the parent node or -1

Approach to Problem Solving

We can use several methods to quickly find the parent node of the given tree. The most commonly used method is through array access. By storing the parent node information in an array, we can find the parent of a specific node in constant time. The process is as follows:

1. Choosing a Data Structure

To effectively store the parent information of nodes, we will use an array. The index of the array corresponds to the value of the node, and the value stored at each index is the value of the parent node. For example, if the parent information of the nodes is as follows:

[-1, 0, 0, 1, 1, 2]

In the above array:

  • Parent of node 0: -1 (root node)
  • Parent of node 1: 0
  • Parent of node 2: 0
  • Parent of node 3: 1
  • Parent of node 4: 1
  • Parent of node 5: 2

2. Designing the Algorithm

Now, we will design the algorithm. The overall process is as follows:

  1. Initialize an array to store the parent information.
  2. Receive the parent information array as input.
  3. Receive the node to be found as input.
  4. Check if the input node is within the range of the array.
  5. Find and return the parent node.
  6. If the parent node is the root node, return -1.

Java Code Implementation

Now, let’s implement the algorithm described above in Java. First, we will define the necessary classes and write the main method:

import java.util.Scanner;

public class FindParentNode {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        // Input the number of nodes
        int n = sc.nextInt();
        int[] parents = new int[n];
        
        // Input parent information
        for (int i = 0; i < n; i++) {
            parents[i] = sc.nextInt();
        }
        
        // Input the target node
        int target = sc.nextInt();
        
        // Find the parent node
        int result = findParent(parents, target);
        System.out.println(result);
        
        sc.close();
    }
    
    // Method to find the parent node
    public static int findParent(int[] parents, int node) {
        if (node < 0 || node >= parents.length) {
            return -1; // Node does not exist
        }
        // Return the parent node
        return parents[node];
    }
}

Explanation

In the above code, we use the Scanner class to read inputs and store the parent information in the parents array. We then input the target node and call the findParent method to find the parent node. This method checks if the input node is valid, then returns the parent of the specified node.

Test Cases

Now, let’s create some test cases to ensure that the implemented code works correctly.

Test Case 1

Input:

6
-1 0 0 1 1 2
3

Output:

1

Test Case 2

Input:

6
-1 0 0 1 1 2
0

Output:

-1

Test Case 3

Input:

6
-1 0 0 1 1 2
6

Output:

-1

Conclusion

Through this posting, we explored the process of solving the problem of finding the parent node of a tree using Java. The tree structure is one of the fundamental concepts in data structures, and understanding the basic parent-child relationships will also be useful for solving other complex tree problems. I hope you will practice various problems and create your own solutions in preparation for coding tests.