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  1. Practice
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  3. Cracking the Coding Interview
  4. Arrays: Left Rotation
  5. Discussions

Arrays: Left Rotation

  • darkOverLord + 6 comments

    I did it this way, in Java

    public static int[] arrayLeftRotation(int[] a, int n, int k) {
        if (k >= n) {
            k = k % n;
        }
        
        if (k == 0) return a;
        
        int[] temp = new int[n];
        
        for (int i = 0; i < n; i++) {
            if (i + k < n) {
                temp[i] = a[i + k];
            } else {
                temp[i] = a[(i + k) - n];
            }
        }
        
        return temp;
    }
    • vinaysh + 1 comment

      instead of if-else statement temp[i] = a[(i+k)%n]; would be enough.

      Also this solution would take up extra memory(for temp).

      • shortcut2alireza + 2 comments

        Would you mind sharing your solution that does it in-place? Thanks

        • jacob0306 + 1 comment

          in case you need it! hope it help!

          import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {
    
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int k = in.nextInt();
        int a[] = new int[n];
        
        for(int a_i=0; a_i < n; a_i++){
            a[((n- (k%n))+a_i)%n] = in.nextInt();
        }
        for(int i = 0; i < n; i++)
            System.out.print(a[i] + " ");
      
        System.out.println();
      
    }
}
          • ngokli + 1 comment

            This is a good reason to start looping on the first element of the input, rather than the output.

            But the problem statement does say you are "Given an array" (not a stream).

            • jacob0306 + 1 comment

              Nah I did not add anything except changed the gived codes from a [a_i] = in.nextInt() to a[((n- (k%n))+a_i)%n] = in.nextInt()

              • adivishy + 0 comments

                can you pl explain a[((n- (k%n))+a_i)%n] = in.nextInt() st

        • hackerrank_com23 + 3 comments

          Here's my in-place function implementation:

          public static int[] arrayLeftRotation(int[] a, int n, int k) {
    // Rotate in-place
    int[] temp = new int[k];
    System.arraycopy(a, 0, temp, 0, k);
    System.arraycopy(a, k, a, 0, n - k);
    System.arraycopy(temp, 0, a, n - k, k);
    return a;
}
          • Angi29 + 0 comments
            [deleted]
          • Angi29 + 0 comments

            idem, just a little variation : kifs

            static int[] rotLeft(int[] a, int d) {
    int length = a.length;
    int[] b = new int[length];

    System.arraycopy(a, d, b, 0, length - d);
    System.arraycopy(a, 0, b, length - d, d);
    return b;
}
          • denis631 + 1 comment

            How is this in-place when you allocate a new array of size k.

            • venom1724 + 0 comments

              Indeed, its not in-place at all. Also its slow. If you are interested, here's an inplace c++ solution 

              void rotLeft(vector<int> &a, int d) {
    int c=a[d],ci=0,n=a.size(); a[d]=0;
    for(int i=n-1;i>=0;i--){
        swap(a[ci],c);
        if(!c&&i) ci+=n-i+++i;
        ci=(n+ci-d)%n;
    }
}
    • mohitgautam2k16 + 0 comments
      [deleted]
    • 123shuklaprasha1 + 0 comments

      why u have used that command of else please explain me

    • cc_insp + 0 comments

      I used the System.arraycopy() method which was used in the video tutorial. I'm wondering if this solution is more efficient or mine?

      import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int k = in.nextInt();
        int a[] = new int[n];
        for(int a_i=0; a_i < n; a_i++){
            a[a_i] = in.nextInt();
        }
        a = leftRotation(n, k, a);
        for (int i=0; i<a.length; i++) {
            System.out.print(a[i]+" ");
        }
    }
    public static int[] leftRotation(int n, int k, int[] a){
        int[] copy = new int[n];
        System.arraycopy(a, k, copy, 0, (n - k));
        System.arraycopy(a, 0, copy, (n - k), (n - (n - k)));
        return copy;
    }
}
    • yash_97373 + 0 comments

      Is calculation really required ?

      for(int i=k; i < a.length; i++){
    System.out.print(a[i] + " ");
}
for(int i=0; i < k; i++){
    System.out.print(a[i] + " ");
}
    • arn_pune + 0 comments
      [deleted]