Rearrange given Array by splitting in half and inserting second half in reverse at alternate position

Given an array arr[] of even length N, the task is to perform the following operations on the given array:

• Split the given array in half.
• Insert the second half in reverse order at alternate positions from the start.

Examples: 

Input: N = 6, arr[] = {1, 2, 3, 4, 5, 6}
Output: 1 6 2 5 3 4
Explanation: The first element is 1.
Then print the last element 6
Then the 2nd element 2 followed by the 2nd last element (i.e. the 5th element) 5.
Now print the 3rd element which is 3 and the 3rd last (i.e. the 4th element) 4.

Input: N = 4, arr[] = {12, 34, 11, 20}
Output: 12 20 34 11

Approach: This problem is solved by splitting the array into two parts with the help of two pointers i starting from 0 and j starting from (N-1). Now print the elements at i and j alternatively and increment i/ decrement j when an element at ith / jth position is inserted in new array.

Below is the implementation of the above approach.

// C++ Program to implement the approach
#include <bits/stdc++.h>
using namespace std;

// Function to arrange array in alternate order
vector<int> printAlternate(int arr[], int N)
{
    int i = 0, j = N - 1;
    vector<int> ans;

    // Run loop while i <= j
    while (i <= j) {

        // Push the element from starting
        ans.push_back(arr[i]);

        // Push the element from back
        ans.push_back(arr[j]);

        // Increment i by 1
        i = i + 1;

        // Decrement j by 1
        j = j - 1;
    }
    return ans;
}

// Driver code
int main()
{
    int N = 6;
    int arr[N] = { 1, 2, 3, 4, 5, 6 };

    // Function call
    vector<int> ans = printAlternate(arr, N);
    for (int x : ans)
        cout << x << " ";
    return 0;
}

// Java Program to implement the approach
import java.util.*;

class GFG {

  // Function to arrange array in alternate order
  public static ArrayList<Integer>
    printAlternate(int[] arr, int N)
  {
    int i = 0, j = N - 1;
    ArrayList<Integer> ans = new ArrayList<>();

    // Run loop while i <= j
    while (i <= j) {

      // Push the element from starting
      ans.add(arr[i]);

      // Push the element from back
      ans.add(arr[j]);

      // Increment i by 1
      i = i + 1;

      // Decrement j by 1
      j = j - 1;
    }
    return ans;
  }

  public static void main(String[] args)
  {
    int N = 6;
    int arr[] = { 1, 2, 3, 4, 5, 6 };

    // Function call
    ArrayList<Integer> ans = printAlternate(arr, N);
    for (int x : ans)
      System.out.print(x + " ");
  }
}

// This code is contributed by ninja_hattori.

# Python Program to implement the approach
# Function to arrange array in alternate order

def printAlternate(arr, N):
    i = 0
    j = N - 1
    ans = []

    # Run loop while i <= j
    while (i <= j):

        # Push the element from starting
        ans.append(arr[i])

        # Push the element from back
        ans.append(arr[j])

        # Increment i by 1
        i = i + 1

        # Decrement j by 1
        j = j - 1

    return ans


# Driver code

N = 6
arr = [1, 2, 3, 4, 5, 6]

# Function call
ans = printAlternate(arr, N)
for x in ans:
    print(x, end=' ')

# This code is contributed by Palak Gupta

// C# Program to implement the approach
using System;
using System.Collections;

class GFG {

  // Function to arrange array in alternate order
  static ArrayList printAlternate(int[] arr, int N)
  {
    int i = 0, j = N - 1;
    ArrayList ans = new ArrayList();

    // Run loop while i <= j
    while (i <= j) {

      // Push the element from starting
      ans.Add(arr[i]);

      // Push the element from back
      ans.Add(arr[j]);

      // Increment i by 1
      i = i + 1;

      // Decrement j by 1
      j = j - 1;
    }
    return ans;
  }

  // Driver code
  public static void Main()
  {
    int N = 6;
    int[] arr = { 1, 2, 3, 4, 5, 6 };

    // Function call
    ArrayList ans = printAlternate(arr, N);
    foreach(int x in ans) Console.Write(x + " ");
  }
}

// This code is contributed by Samim Hossain Mondal.

<script>
        // JavaScript code for the above approach

        // Function to arrange array in alternate order
        function printAlternate(arr, N)
        {
            let i = 0, j = N - 1;
            let ans = [];

            // Run loop while i <= j
            while (i <= j) {

                // Push the element from starting
                ans.push(arr[i]);

                // Push the element from back
                ans.push(arr[j]);

                // Increment i by 1
                i = i + 1;

                // Decrement j by 1
                j = j - 1;
            }
            return ans;
        }

        // Driver code

        let N = 6;
        let arr = [1, 2, 3, 4, 5, 6];

        // Function call
        let ans = printAlternate(arr, N);
        for (let x of ans)
            document.write(x + " ");

       // This code is contributed by Potta Lokesh
    </script>

1 6 2 5 3 4 

Time Complexity: O(N)
Auxiliary Space: O(N)

nobita04

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Article Tags :

• Misc
• DSA
• Arrays
• array-rearrange
• Reverse
• partition

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Practice Tags :

• Arrays
• Misc
• Reverse