Center-Outward Traversal of arrays in Java

Introduction

Greetings! Welcome to our lesson today where we'll unravel a fascinating aspect of array manipulation . Here's the question: How would you traverse an array not from the beginning to the end, or vice versa, but from the center outward in either direction? Today's lesson is all about exploring this concept with Java. Brace yourself for a captivating learning journey.

Task Statement

Our task is to produce a new array, given an array of integers, that starts from the center of the original array and alternates direction towards both ends. That is, the first element of our new array will be the middle element of the original one. After defining the starting point, we will alternate between elements to the left and to the right of this center until all elements have been included. If the length of the initial array is even, we first take the element to the left of the center, then the one to the right of the center, then do the alternation as described above. For example, for numbers = [1, 2, 3, 4, 5], the output would be [3, 2, 4, 1, 5] . We will break down this seemingly complex task into manageable pieces to progressively build our Java solution. Keep in mind an additional condition: the length of the array — represented as n — can range from 1 to 100,000, inclusive.

Solution Building: Step 1

First, let's establish the midpoint of our array. Our task requires us to expand our array from the center to the ends, so we divide its length by 2 using integer division in Java. If we find that the array's length is odd, we include the middle element in the newOrder array, given it has no counterpart. If the array's length is even, newOrder initially remains empty. Here is how it looks in Java: Java import java.util.ArrayList; public class CenterOutwardTraversal {public static ArrayList<Integer> iterateMiddleToEnd(ArrayList<Integer> numbers) {int mid = numbers.size() / 2; // index of the left middle element ArrayList<Integer> newOrder = new ArrayList<>(); // ArrayList to store new order if (numbers.size() % 2 == 1) {newOrder.add(numbers.get(mid)); // Adding middle element to resulting ArrayList if length is odd} // newOrder remains empty for now if length is even return newOrder;}}

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Solution Building: Step 3

With our pointers initialized, it's time to navigate the array and form our new order. In Java, while loops provide the necessary control structure to iterate from the center of the array to both ends. In each iteration, we push the elements at indices left and right to the newOrder array, decrement left by one and increment right by one.

Here's how it looks when we put together the full Java method:

Java

import java.util.ArrayList;
import java.util.Arrays;

public class CenterOutwardTraversal {
    public static ArrayList<Integer> iterateMiddleToEnd(ArrayList<Integer> numbers) {
        int mid = numbers.size() / 2; // index of the left middle element
        int left, right;
        ArrayList<Integer> newOrder = new ArrayList<>();  // ArrayList to store new order

        if (numbers.size() % 2 == 1) {
            left = mid - 1; // Pointing to the left of the middle element
            right = mid + 1; // Pointing to the right of the middle element
            newOrder.add(numbers.get(mid)); // Adding the middle element to the resulting ArrayList
        } else {
            left = mid - 1; // Pointing to the left of the middle element
            right = mid; // Pointing to the middle element
        }

        while (left >= 0 && right < numbers.size()) {
            newOrder.add(numbers.get(left--));
            newOrder.add(numbers.get(right++));
        }

        return newOrder;
    }

    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
        
        ArrayList<Integer> result = iterateMiddleToEnd(numbers);
        System.out.println(result);  // Output should be [3, 2, 4, 1, 5]
    }
}

import java.util.ArrayList;
import java.util.Arrays;

public class CenterOutwardTraversal {
    public static ArrayList<Integer> iterateMiddleToEnd(ArrayList<Integer> numbers) {
        int mid = numbers.size() / 2; // index of the left middle element
        int left, right;
        ArrayList<Integer> newOrder = new ArrayList<>();  // ArrayList to store new order

        if (numbers.size() % 2 == 1) {
            left = mid - 1; // Pointing to the left of the middle element
            right = mid + 1; // Pointing to the right of the middle element
            newOrder.add(numbers.get(mid)); // Adding the middle element to the resulting ArrayList
        } else {
            left = mid - 1; // Pointing to the left of the middle element
            right = mid; // Pointing to the middle element
        }

        while (left >= 0 && right < numbers.size()) {
            newOrder.add(numbers.get(left--));
            newOrder.add(numbers.get(right++));
        }

        return newOrder;
    }

    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5));
        
        ArrayList<Integer> result = iterateMiddleToEnd(numbers);
        System.out.println(result);  // Output should be [3, 2, 4, 1, 5]
    }
}

By implementing this approach, we have successfully created a new array, starting from the middle and alternating to the left and right ends of the original array, effectively fulfilling the requirements of our task!

Introduction

Task Statement

Solution Building: Step 1

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Solution Building: Step 2

Solution Building: Step 3

Lesson Summary