Lesson 4

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.

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.

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`1import java.util.ArrayList; 2 3public class CenterOutwardTraversal { 4 public static ArrayList<Integer> iterateMiddleToEnd(ArrayList<Integer> numbers) { 5 int mid = numbers.size() / 2; // index of the left middle element 6 ArrayList<Integer> newOrder = new ArrayList<>(); // ArrayList to store new order 7 8 if (numbers.size() % 2 == 1) { 9 newOrder.add(numbers.get(mid)); // Adding middle element to resulting ArrayList if length is odd 10 } 11 // newOrder remains empty for now if length is even 12 13 return newOrder; 14 } 15}`

Successfully solving our task requires two pointers: `left`

and `right`

. These pointers are initialized to point to the elements immediately to the left and right of the middle element, respectively.

Here is the Java method with the added initialization of these pointers:

Java`1import java.util.ArrayList; 2 3public class CenterOutwardTraversal { 4 public static ArrayList<Integer> iterateMiddleToEnd(ArrayList<Integer> numbers) { 5 int mid = numbers.size() / 2; // index of the left middle element 6 int left, right; 7 ArrayList<Integer> newOrder = new ArrayList<>(); // ArrayList to store new order 8 9 if (numbers.size() % 2 == 1) { 10 left = mid - 1; // Pointing to the left of the middle element 11 right = mid + 1; // Pointing to the right of the middle element 12 newOrder.add(numbers.get(mid)); // Adding the middle element to the resulting ArrayList 13 } else { 14 left = mid - 1; // Pointing to the left of the middle element 15 right = mid; // Pointing to the middle element 16 } 17 18 return newOrder; 19 } 20}`

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`1import java.util.ArrayList; 2import java.util.Arrays; 3 4public class CenterOutwardTraversal { 5 public static ArrayList<Integer> iterateMiddleToEnd(ArrayList<Integer> numbers) { 6 int mid = numbers.size() / 2; // index of the left middle element 7 int left, right; 8 ArrayList<Integer> newOrder = new ArrayList<>(); // ArrayList to store new order 9 10 if (numbers.size() % 2 == 1) { 11 left = mid - 1; // Pointing to the left of the middle element 12 right = mid + 1; // Pointing to the right of the middle element 13 newOrder.add(numbers.get(mid)); // Adding the middle element to the resulting ArrayList 14 } else { 15 left = mid - 1; // Pointing to the left of the middle element 16 right = mid; // Pointing to the middle element 17 } 18 19 while (left >= 0 && right < numbers.size()) { 20 newOrder.add(numbers.get(left--)); 21 newOrder.add(numbers.get(right++)); 22 } 23 24 return newOrder; 25 } 26 27 public static void main(String[] args) { 28 ArrayList<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5)); 29 30 ArrayList<Integer> result = iterateMiddleToEnd(numbers); 31 System.out.println(result); // Output should be [3, 2, 4, 1, 5] 32 } 33}`

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!

Well done on reaching the end of this lesson! You've uncovered a riveting method of traversing and manipulating array! Take a moment to pat yourself on the back for understanding and implementing this concept. As the proverb goes, practice makes perfect. Hence, I encourage you to employ this concept in similar problems. Your exceptional journey in mastering algorithms with Java has just begun. Embrace the challenge and happy coding!