BlockSwapRecursion.java

package ArrayChallenges;

import java.util.Scanner;


import java.util.*;

class BlockSwapRecursion {
    // Wrapper over the recursive function leftRotateRec()
    // It left rotates arr[] by d.
    public static void leftRotate(int arr[], int d,
                                  int n) {
        leftRotateRec(arr, 0, d, n);
    }

    public static void leftRotateRec(int arr[], int i,
                                     int d, int n) {
        /* Return If number of elements to be rotated
        is zero or equal to array size */
        if (d == 0 || d == n)
            return;

        /*If number of elements to be rotated
        is exactly half of array size */
        if (n - d == d) {
            swap(arr, i, n - d + i, d);
            return;
        }

        /* If A is shorter*/
        if (d < n - d) {
            swap(arr, i, n - d + i, d);
            leftRotateRec(arr, i, d, n - d);
        } else /* If B is shorter*/ {
            swap(arr, i, d, n - d);
            leftRotateRec(arr, n - d + i, 2 * d - n, d); /*This is tricky*/
        }
    }

    /*UTILITY FUNCTIONS*/
    /* function to print an array */
    public static void printArray(int arr[], int size) {
        int i;
        for (i = 0; i < size; i++)
            System.out.print(arr[i] + " ");
        System.out.println();
    }

    /*This function swaps d elements
    starting at index fi with d elements
    starting at index si */
    public static void swap(int arr[], int fi,
                            int si, int d) {
        int i, temp;
        for (i = 0; i < d; i++) {
            temp = arr[fi + i];
            arr[fi + i] = arr[si + i];
            arr[si + i] = temp;
        }
    }

    // Driver Code
    public static void main(String[] args) {
        int arr[] = {1, 2, 3, 4, 5, 6, 7};
        leftRotate(arr, 2, 7);
        printArray(arr, 7);
    }
}

// This code is contributed by codeseeker