(7 marks) Create a void method called my_reverse with the following signature: p

Question

(7 marks) Create a void method called my_reverse with the following signature:

public static void my_reverse(int[] arr, int start, int end)

Your method should reverse the elements of the sub-array from indexes start to end, inclusive. No need to create a new array, just modify the array that was passed in. You may not use any existing reverse() functionality in the Java library. You must implement the reversal yourself.

For example: If the array is {2, 4, 6, 5, 3}, and you call my_reverse(array, 1, 3), The new ordering of the array should be: {2, 5, 6, 4, 3}

An iterative solution will earn a maximum of 5 marks. To achieve the full 7 marks, your solution must be recursive. If the indexes are invalid in any way, do nothing to the array.

Explanation / Answer

Recursive functions are those functions which call themselves with different parameters in every call. They should always have a bound check where the recursion will break otherwise the functions will be called infinitely.

The following method calls the same method with the start index after increasing by 1 and end index after decreasing by 1 and breaks when start >= end or value of start or end is invalid. This means we call my_reverse() with the sub array of 2 less elements and then we swap the start and end indices elements. For my_reverse(arr, 1, 3), my_reverse(arr, 2, 2) is called. In my_reverse(arr, 2, 2), as 2 >= 2, it returns without calling any other function and back in my_reverse(arr, 1, 3), arr[1] and arr[3] are swapped and then control is returned to main(). You can do it in your notebook for better understanding and to avoid confusion.

public static void my_reverse(int[] arr, int start, int end)

{

       //Return to parent function, if start or end value is invalid

       if(start < 0 || start > arr.length - 1 || end < 0 || end > arr.length - 1)

             return;

       //Return to parent function if start is equal or greater than end

       if(start >= end)

             return;

       //Following statements will only run if both the above conditions are false

       //Call my_reverse() with same array, increased start and decreased end

       my_reverse(arr, start + 1, end - 1);

       //Swap arr[start] and arr[end]

       int temp = arr[start];

       arr[start] = arr[end];

       arr[end] = temp;

}

The function definition is thoroughly checked for different valid and invalid test cases.

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