Reverse an array in groups of given size | Set 3 (Single traversal)


Given an array, reverse every sub-array formed by consecutive k elements.

Examples:

Input: arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], k = 3.
Output: [3, 2, 1, 6, 5, 4, 9, 8, 7, 10]

Input: arr = [1, 2, 3, 4, 5, 6, 7], k = 5.
Output: [5, 4, 3, 2, 1, 7, 6]

Approach:



  1. We will use two pointers technique to solve this problem.
  2. First, we will initialize our 1st pointer d with value k-1 (d = k-1) and a variable m with value 2 (m = 2).
  3. Now, we will iterate the array with our 2nd pointer i and check

    • If i < d, Swap (arr[i], arr[d]) and decrement d by 1. Otherwise,
    • Make d = k * m – 1, i = k * (m – 1) – 1 and m = m + 1.

Below is the implementation of the above approach.

C++

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// C++ program to reverse every sub-array
// formed by consecutive k elements
#include<bits/stdc++.h>
using namespace std;
  
// Function to reverse every sub-array 
// formed by consecutive k elements 
void ReverseInGroup(int arr[], int n, int k)
{
    if(n < k)
    {
        k = n;
    }
  
    // Initialize variables
    int d = k - 1, m = 2;
    int i = 0;
          
    for(i = 0; i < n; i++)
    {
         
       if (i >= d)
       
           // Update the variables
           d = k * (m);
           if(d >= n)
           {
               d = n;         
           }     
           i = k * (m - 1) - 1;
           m++;
       }
       else
       {
           int t = arr[i];
           arr[i] = arr[d];
           arr[d] = t;
       }
       d = d - 1; 
    }
    return;
}
  
// Driver code 
int main() 
    int arr[] = { 1, 2, 3, 4, 5, 6, 7 }; 
    int k = 3; 
    int n = sizeof(arr) / sizeof(arr[0]); 
  
    ReverseInGroup(arr, n, k); 
  
    for(int i = 0; i < n; i++) 
       cout << arr[i] << " "
  
    return 0; 
}
  
// This code is contributed by Code_Mech

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C

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// C program to reverse every sub-array
// formed by consecutive k elements
#include<stdio.h>
  
// Function to reverse every sub-array 
// formed by consecutive k elements 
void ReverseInGroup(int arr[], int n, int k)
{
    if(n<k)
    {
       k=n;
    }
  
    // Initialize variables
    int d = k-1, m=2;
    int i = 0;
          
    for (i = 0; i < n; i++)
    {
        if (i >= d)
        {   
            // Update the variables
            d = k * (m);
            if(d>=n)
            {
                d = n;             
            }           
            i = k * (m - 1)-1;
            m++;
              
        }
        else
        {
            int t = arr[i];
            arr[i] = arr[d];
            arr[d] = t;
        }
  
     d = d - 1;      
         
    }
    return;
          
}
  
// Driver code 
int main() 
    int arr[] = {1, 2, 3, 4, 5, 6, 7}; 
    int k = 3; 
    
    int n = sizeof(arr) / sizeof(arr[0]); 
    
    ReverseInGroup(arr, n, k); 
    
    for (int i = 0; i < n; i++) 
        printf("%d ", arr[i]); 
    
    return 0; 
}

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Java

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// Java program to reverse every sub-array
// formed by consecutive k elements
class GFG{
      
// Function to reverse every sub-array 
// formed by consecutive k elements 
static void ReverseInGroup(int arr[], 
                           int n, int k)
{
    if(n < k)
    {
        k = n;
    }
  
    // Initialize variables
    int d = k - 1, m = 2;
    int i = 0;
          
    for(i = 0; i < n; i++)
    {
       if (i >= d)
       
             
           // Update the variables
           d = k * (m);
           if(d >= n)
           {
               d = n;             
           }         
           i = k * (m - 1) - 1;
           m++;
       }
       else
       {
           int t = arr[i];
           arr[i] = arr[d];
           arr[d] = t;
       }
       d = d - 1;     
    }
    return;
}
  
// Driver code 
public static void main(String[] args) 
{
    int arr[] = { 1, 2, 3, 4, 5, 6, 7 }; 
    int k = 3
    int n = arr.length; 
      
    ReverseInGroup(arr, n, k); 
      
    for(int i = 0; i < n; i++) 
       System.out.printf("%d ", arr[i]); 
}
  
// This code is contributed by sapnasingh4991

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C#

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// C# program to reverse every sub-array
// formed by consecutive k elements
using System;
class GFG{
      
// Function to reverse every sub-array 
// formed by consecutive k elements 
static void ReverseInGroup(int []arr, 
                           int n, int k)
{
    if(n < k)
    {
        k = n;
    }
  
    // Initialize variables
    int d = k - 1, m = 2;
    int i = 0;
          
    for(i = 0; i < n; i++)
    {
        if (i >= d)
        
                  
            // Update the variables
            d = k * (m);
            if(d >= n)
            {
                d = n;             
            }         
            i = k * (m - 1) - 1;
            m++;
        }
        else
        {
            int t = arr[i];
            arr[i] = arr[d];
            arr[d] = t;
        }
        d = d - 1;     
    }
    return;
}
  
// Driver code 
public static void Main() 
{
    int []arr = { 1, 2, 3, 4, 5, 6, 7 }; 
    int k = 3; 
    int n = arr.Length; 
      
    ReverseInGroup(arr, n, k); 
      
    for(int i = 0; i < n; i++) 
        Console.Write(arr[i] + " "); 
}
  
// This code is contributed by Code_Mech

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Output:

3 2 1 6 5 4 7

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Improved By : sapnasingh4991, Code_Mech