# Array obtained by repeatedly reversing array after every insertion from given array

Given an array arr[], the task is to print the array obtained by inserting elements of arr[] one by one into an initially empty array, say arr1[], and reversing the array arr1[] after every insertion.

Examples:

Input: arr[] = {1, 2, 3, 4}
Output: 4 2 1 3
Explanation:
Operations performed on the array arr1[] as follows:
Step 1: Append 1 into the array and reverse it. arr1[] = {1}
Step 2: Append 2 into the array and reverse it. arr1[] = {2, 1}
Step 3: Append 3 into the array and reverse it. arr1[] = {3, 1, 2}
Step 3: Append 4 into the array and reverse it. arr1[] = {4, 2, 1, 3}

Input: arr[]  = {1, 2, 3}
Output: 3 1 2
Explanation:
Operations performed on the array arr1[] as follows:
Step 1: Append 1 into the array and reverse it. arr1[] = {1}
Step 2: Append 2 into the array and reverse it. arr1[] = {2, 1}
Step 3: Append 3 into the array and reverse it. arr1[] = {3, 1, 2}

Naive Approach: The simplest approach to solve the problem is to iterate over the array arr[] and insert each element of arr[] one by one into the array arr1[] and reverse the array arr1[] after each insertion.

Time Complexity: O(N2)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, the idea is to use the Doubly Ended Queue (Deque) to append the elements at both ends. Follow the steps below to solve the problem:

Below is the implementation of the above approach:

## C++

 `// C++ program of the above approach`   `#include ` `using` `namespace` `std;`   `// Function to generate the array by` `// inserting array elements one by one` `// followed by reversing the array` `void` `generateArray(``int` `arr[], ``int` `n)` `{`   `    ``// Doubly ended Queue` `    ``deque<``int``> ans;`   `    ``// Iterate over the array` `    ``for` `(``int` `i = 0; i < n; i++) {`   `        ``// Push array elements` `        ``// alternately to the front` `        ``// and back` `        ``if` `(i & 1)` `            ``ans.push_front(arr[i]);` `        ``else` `            ``ans.push_back(arr[i]);` `    ``}`   `    ``// If size of list is odd` `    ``if` `(n & 1) {`   `        ``// Reverse the list` `        ``reverse(ans.begin(),` `                ``ans.end());` `    ``}`   `    ``// Print the elements` `    ``// of the array` `    ``for` `(``auto` `x : ans) {` `        ``cout << x << ``" "``;` `    ``}` `    ``cout << endl;` `}`   `// Driver Code` `int32_t main()` `{` `    ``int` `n = 4;` `    ``int` `arr[n] = { 1, 2, 3, 4 };` `    ``generateArray(arr, n);` `    ``return` `0;` `}`

## Java

 `// Java program of the above approach ` `import` `java.io.*;` `import` `java.util.*;`   `class` `GFG{` `    `  `// Function to generate the array by` `// inserting array elements one by one` `// followed by reversing the array` `static` `void` `generateArray(``int` `arr[], ``int` `n)` `{` `    `  `    ``// Doubly ended Queue` `    ``Deque ans = ``new` `LinkedList<>();` ` `  `    ``// Iterate over the array` `    ``for``(``int` `i = ``0``; i < n; i++) ` `    ``{` `        `  `        ``// Push array elements` `        ``// alternately to the front` `        ``// and back` `        ``if` `((i & ``1``) != ``0``)` `            ``ans.addFirst(arr[i]);` `        ``else` `            ``ans.add(arr[i]);` `    ``}` ` `  `    ``// If size of list is odd` `    ``if` `((n & ``1``) != ``0``) ` `    ``{` `        `  `        ``// Reverse the list` `        ``Collections.reverse(Arrays.asList(ans)); ` `    ``}` ` `  `    ``// Print the elements` `    ``// of the array` `    ``for``(``int` `x : ans)` `    ``{` `        ``System.out.print(x + ``" "``);` `    ``}` `    ``System.out.println();` `}` ` `  `// Driver Code` `public` `static` `void` `main (String[] args)` `{` `    ``int` `n = ``4``;` `    ``int` `arr[] = { ``1``, ``2``, ``3``, ``4` `};` `    `  `    ``generateArray(arr, n);` `}` `}`   `// This code is contributed by code_hunt`

## Python3

 `# Python3 program of the above approach ` `from` `collections ``import` `deque`   `# Function to generate the array by` `# inserting array elements one by one` `# followed by reversing the array` `def` `generateArray(arr, n):` ` `  `    ``# Doubly ended Queue` `    ``ans ``=` `deque()` ` `  `    ``# Iterate over the array` `    ``for` `i ``in` `range``(n):` ` `  `        ``# Push array elements` `        ``# alternately to the front` `        ``# and back` `        ``if` `(i & ``1` `!``=` `0``):` `            ``ans.appendleft(arr[i])` `        ``else``:` `            ``ans.append(arr[i])` `    `  `    ``# If size of list is odd` `    ``if` `(n & ``1` `!``=` `0``):` ` `  `        ``# Reverse the list` `        ``ans.reverse()` `    `  `    ``# Print the elements` `    ``# of the array` `    ``for` `x ``in` `ans:` `        ``print``(x, end ``=` `" "``)` `    `  `    ``print``()`   `# Driver Code` `n ``=` `4` `arr ``=` `[ ``1``, ``2``, ``3``, ``4` `]`   `generateArray(arr, n)`   `# This code is contributed by code_hunt`

## C#

 `// C# program of ` `// the above approach ` `using` `System;` `using` `System.Collections.Generic;` `class` `GFG{` `    `  `// Function to generate the array ` `// by inserting array elements ` `// one by one followed by ` `// reversing the array` `static` `void` `generateArray(``int` `[]arr, ` `                          ``int` `n)` `{    ` `  ``// Doubly ended Queue` `  ``List<``int``> ans = ``new` `List<``int``>();`   `  ``// Iterate over the array` `  ``for``(``int` `i = 0; i < n; i++) ` `  ``{` `    ``// Push array elements` `    ``// alternately to the front` `    ``// and back` `    ``if` `((i & 1) != 0)` `      ``ans.Insert(0, arr[i]);` `    ``else` `      ``ans.Add(arr[i]);` `  ``}`   `  ``// If size of list is odd` `  ``if` `((n & 1) != 0) ` `  ``{` `    ``// Reverse the list` `    ``ans.Reverse(); ` `  ``}`   `  ``// Print the elements` `  ``// of the array` `  ``foreach``(``int` `x ``in` `ans)` `  ``{` `    ``Console.Write(x + ``" "``);` `  ``}` `  ``Console.WriteLine();` `}` ` `  `// Driver Code` `public` `static` `void` `Main(String[] args)` `{` `  ``int` `n = 4;` `  ``int` `[]arr = {1, 2, 3, 4};` `  ``generateArray(arr, n);` `}` `}`   `// This code is contributed by 29AjayKumar`

Output:

```4 2 1 3

```

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

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