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Reverse the elements only at odd positions in the given Array
• Last Updated : 10 May, 2021

Given an array arr[] containing N integers, the task is to rearrange the array such that the odd indexed elements are in reverse order.

Examples:

Input: arr[] = {5, 7, 6, 2, 9, 18, 11, 15}
Output: {5, 15, 6, 18, 9, 2, 11, 7}
Explanation:
The elements at even index [5, 6, 9, 11] are unchanged and elements at odd index are reversed from [7, 2, 18, 15] to [15, 18, 2, 7].

Input: arr[] = {1, 2, 3, 4, 5, 6}
Output: {1, 6, 3, 4, 5, 2}
Explanation:
The elements at even index are unchanged and elements at odd index are reversed from [2, 4, 6] to [6, 4, 2].

Approach: To solve the problem mentioned above follow the steps given below:

• Push the elements of odd indexes of the given array into a stack data structure.
• Replace the current array elements at odd indexes with elements at top of the stack and keep popping until the stack is empty.

Below is the implementation of the above approach:

## C++

 `// C++ program to reverse the``// elements only at odd positions``// in the given Array` `#include ``using` `namespace` `std;` `// Function to display elements``void` `show(``int` `arr[], ``int` `n)``{``    ``cout << ``"{"``;` `    ``for` `(``int` `i = 0; i < n - 1; i++)``        ``cout << arr[i] << ``", "``;` `    ``cout << arr[n - 1] << ``"}"``;``}` `// Function to flip elements``// at odd indexes``void` `flipHalf(``int` `arr[], ``int` `n)``{``    ``int` `c = 0;``    ``int` `dup = n;` `    ``stack<``int``> st;` `    ``// Pushing elements at odd indexes``    ``// of a array to a stack``    ``for` `(``int` `i = 0; i < n; i++) {``        ``int` `x = arr[i];` `        ``if` `(c % 2 == 1)``            ``st.push(x);``        ``c++;``    ``}` `    ``c = 0;` `    ``// Replacing current elements at odd``    ``// indexes with element at top of stack``    ``for` `(``int` `i = 0; i < n; i++) {``        ``int` `x = arr[i];` `        ``if` `(c % 2 == 1) {``            ``x = st.top();``            ``st.pop();``        ``}``        ``arr[i] = x;``        ``c++;``    ``}``}` `// Driver Code``int` `main()``{``    ``int` `arr[] = { 1, 2, 3, 4, 5, 6 };` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);` `    ``flipHalf(arr, n);` `    ``show(arr, n);` `    ``return` `0;``}`

## Java

 `// Java program to reverse the``// elements only at odd positions``// in the given array``import` `java.io.*;``import` `java.util.*;` `class` `GFG {` `// Function to count the valley points``// in the given character array``static` `void` `show(``int` `arr[], ``int` `n)``{``    ``System.out.print(``"{"``);``    ` `    ``for``(``int` `i = ``0``; i < n - ``1``; i++)``       ``System.out.print(arr[i] + ``", "``);` `    ``System.out.print(arr[n - ``1``] + ``"}"``);        ``}` `// Function to flip elements``// at odd indexes``public` `static` `void` `flipHalf(``int` `arr[], ``int` `n)``{``    ``int` `c = ``0``;``    ``int` `dup = n;``    ``Stack st = ``new` `Stack<>();``    ` `    ``// Pushing elements at odd indexes``    ``// of a array to a stack``    ``for``(``int` `i = ``0``; i < n; i++)``    ``{``       ``int` `x = arr[i];``       ` `       ``if` `(c % ``2` `== ``1``)``       ``{``           ``st.push(x);``       ``}``       ``c++;``    ``}``    ``c = ``0``;``    ` `    ``// Replacing current elements at odd``    ``// indexes with element at top of stack``    ``for``(``int` `i = ``0``; i < n; i++)``    ``{``       ``int` `x = arr[i];``       ` `       ``if` `(c % ``2` `== ``1``)``       ``{``           ``x = st.peek();``           ``st.pop();``       ``}``       ``arr[i] = x;``       ``c++;``    ``}``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ``int` `arr[] = { ``1``, ``2``, ``3``, ``4``, ``5``, ``6` `};``    ``int` `n = arr.length;``    ` `    ``flipHalf(arr, n);``    ``show(arr, n);``}    ``}` `// This code is contributed by Aman Kumar 27`

## Python3

 `# Python3 program to reverse the``# elements only at odd positions``# in the given Array` `# Function to get top of the stack``def` `peek_stack(stack):``    ` `    ``if` `stack:``        ``return` `stack[``-``1``]` `# Function to display elements``def` `show(arr, n):``    ` `    ``print``(``"{"``, end ``=` `" "``)``    ``for` `i ``in` `range``(``0``, n ``-` `1``):``        ``print``(arr[i], ``","``, end ``=` `" "``)``    ` `    ``print``(arr[n ``-` `1``] , ``"}"``)` `# Function to flip elements``# at odd indexes``def` `flipHalf(arr, n):``    ` `    ``c ``=` `0``    ``dup ``=` `n``    ``stack ``=` `[]``    ` `    ``# Pushing elements at odd indexes``    ``# of a array to a stack``    ``for` `i ``in` `range``(``0``, n):``        ``x ``=` `arr[i]``        ` `        ``if` `c ``%` `2` `=``=` `1``:``            ``stack.append(x)``            ` `        ``c ``=` `c ``+` `1``        ` `    ``c ``=` `0` `    ``# Replacing current elements at odd``    ``# indexes with element at top of stack``    ``for` `i ``in` `range``(``0``, n):``        ``x ``=` `arr[i]``        ` `        ``if` `c ``%` `2` `=``=` `1``:``            ``x ``=` `peek_stack(stack)``            ``stack.pop()``    ` `        ``arr[i] ``=` `x``        ``c ``=` `c ``+` `1` `# Driver Code``if` `__name__ ``=``=` `"__main__"``:``    ` `  ``arr ``=` `[ ``1``, ``2``, ``3``, ``4``, ``5``, ``6` `]``  ``n ``=` `len``(arr)` `  ``flipHalf(arr, n)` `  ``show(arr, n)` `# This code is contributed by akhilsaini`

## C#

 `// C# program to reverse the``// elements only at odd positions``// in the given array``using` `System;``using` `System.Collections.Generic;` `class` `GFG{` `// Function to count the valley points``// in the given character array``static` `void` `show(``int` `[]arr, ``int` `n)``{``    ``Console.Write(``"{"``);``    ` `    ``for``(``int` `i = 0; i < n - 1; i++)``    ``Console.Write(arr[i] + ``", "``);` `    ``Console.Write(arr[n - 1] + ``"}"``);        ``}` `// Function to flip elements``// at odd indexes``public` `static` `void` `flipHalf(``int` `[]arr, ``int` `n)``{``    ``int` `c = 0;``    ``int` `dup = n;``    ``Stack<``int``> st = ``new` `Stack<``int``>();``    ` `    ``// Pushing elements at odd indexes``    ``// of a array to a stack``    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``int` `x = arr[i];``            ` `        ``if` `(c % 2 == 1)``        ``{``            ``st.Push(x);``        ``}``        ``c++;``    ``}``    ``c = 0;``    ` `    ``// Replacing current elements at odd``    ``// indexes with element at top of stack``    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``int` `x = arr[i];``            ` `        ``if` `(c % 2 == 1)``        ``{``            ``x = st.Peek();``            ``st.Pop();``        ``}``        ``arr[i] = x;``        ``c++;``    ``}``}` `// Driver Code``public` `static` `void` `Main(String[] args)``{``    ``int` `[]arr = { 1, 2, 3, 4, 5, 6 };``    ``int` `n = arr.Length;``    ` `    ``flipHalf(arr, n);``    ``show(arr, n);``}    ``}` `// This code is contributed by 29AjayKumar`

## Javascript

 ``
Output:
`{1, 6, 3, 4, 5, 2}`

Time complexity: O(N)
Auxiliary Space Complexity: O(N/2)

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