Rearrange array by interchanging positions of even and odd elements in the given array

Difficulty Level : Basic
Last Updated : 03 May, 2021

Given an array arr[] of N positive integers with an equal number of even and odd elements. The task is to use in-place swapping to interchange positions of even and odd elements in the array.

Examples:

Input: arr[] = {1, 3, 2, 4}
Output: 2 4 1 3
Explanation:
Before rearranging the given array, indices 0 and 1 had odd elements and indices 2 and 3 had even elements.
After rearrangement, array becomes {2, 4, 1, 3} where indices 0 and 1 have even elements and indices 2 and 3 have odd elements.
Input: arr[] = {2, 2, 1, 3}
Output: 1 3 2 2
Explanation:
Before rearranging the given array, indices 0 and 1 had even elements and indices 2 and 3 had odd elements.
After rearrangement, array becomes {1, 3, 2, 2} where indices 0 and 1 have odd elements and indices 2 and 3 have even elements.

Naive Approach: The simplest approach is to iterate over array elements using two loops, the outer loop picks each element of the array and the inner loop is to find the opposite parity element for the picked element and swap them. After swapping elements, mark the picked element as negative so as not to pick it again. Finally, make all elements positive and print the array.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include<bits/stdc++.h>
using namespace std;
 
// Function to replace each even
// element by odd and vice-versa
// in a given array
void replace(int arr[], int n)
{
     
    // Traverse array
    for(int i = 0; i < n; i++)
    {
        for(int j = i + 1; j < n; j++)
        {
             
            // If current element is even
            // then swap it with odd
            if (arr[i] >= 0 && arr[j] >= 0 &&
                arr[i] % 2 == 0 &&
                arr[j] % 2 != 0)
            {
                 
                // Perform Swap
                int tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
 
                // Change the sign
                arr[j] = -arr[j];
 
                break;
            }
 
            // If current element is odd
            // then swap it with even
            else if (arr[i] >= 0 && arr[j] >= 0 &&
                     arr[i] % 2 != 0 &&
                     arr[j] % 2 == 0)
            {
                 
                // Perform Swap
                int tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
 
                // Change the sign
                arr[j] = -arr[j];
 
                break;
            }
        }
    }
 
    // Marked element positive
    for(int i = 0; i < n; i++)
        arr[i] = abs(arr[i]);
 
    // Print final array
    for(int i = 0; i < n; i++)
         cout << arr[i] << " ";
}
 
// Driver Code
int main()
{
     
    // Given array arr[]
    int arr[] = { 1, 3, 2, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
     
    // Function Call
    replace(arr,n);
}
 
// This code is contributed by SURENDRA_GANGWAR

Java

// Java program for the above approach
import java.util.*;
import java.lang.*;
 
class GFG {
 
    // Function to replace each even
    // element by odd and vice-versa
    // in a given array
    static void replace(int[] arr)
    {
        // Stores length of array
        int n = arr.length;
 
        // Traverse array
        for (int i = 0; i < n; i++) {
 
            for (int j = i + 1; j < n; j++) {
 
                // If current element is even
                // then swap it with odd
                if (arr[i] >= 0
                    && arr[j] >= 0
                    && arr[i] % 2 == 0
                    && arr[j] % 2 != 0) {
 
                    // Perform Swap
                    int tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
 
                    // Change the sign
                    arr[j] = -arr[j];
 
                    break;
                }
 
                // If current element is odd
                // then swap it with even
                else if (arr[i] >= 0
                         && arr[j] >= 0
                         && arr[i] % 2 != 0
                         && arr[j] % 2 == 0) {
 
                    // Perform Swap
                    int tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
 
                    // Change the sign
                    arr[j] = -arr[j];
 
                    break;
                }
            }
        }
 
        // Marked element positive
        for (int i = 0; i < n; i++)
            arr[i] = Math.abs(arr[i]);
 
        // Print final array
        for (int i = 0; i < n; i++)
            System.out.print(arr[i] + " ");
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Given array arr[]
        int[] arr = { 1, 3, 2, 4 };
 
        // Function Call
        replace(arr);
    }
}

Python3

# Python3 program for the
# above approach
 
# Function to replace each even
# element by odd and vice-versa
# in a given array
def replace(arr,  n):
 
    # Traverse array
    for i in range(n):
        for j in range(i + 1, n):
 
            # If current element is
            # even then swap it with odd
            if (arr[i] >= 0 and
                arr[j] >= 0 and
                arr[i] % 2 == 0 and
                arr[j] % 2 != 0):
 
                # Perform Swap
                tmp = arr[i]
                arr[i] = arr[j]
                arr[j] = tmp
 
                # Change the sign
                arr[j] = -arr[j]
 
                break
 
            # If current element is odd
            # then swap it with even
            elif (arr[i] >= 0 and
                  arr[j] >= 0 and
                  arr[i] % 2 != 0 and
                  arr[j] % 2 == 0):
 
                # Perform Swap
                tmp = arr[i]
                arr[i] = arr[j]
                arr[j] = tmp
 
                # Change the sign
                arr[j] = -arr[j]
 
                break
 
    # Marked element positive
    for i in range(n):
        arr[i] = abs(arr[i])
 
    # Print final array
    for i in range(n):
        print(arr[i], end = " ")
 
# Driver Code
if __name__ == "__main__":
 
    # Given array arr[]
    arr = [1, 3, 2, 4]
    n = len(arr)
 
    # Function Call
    replace(arr, n)
 
# This code is contributed by Chitranayal

C#

// C# program for the above approach
using System;
 
class GFG{
     
// Function to replace each even
// element by odd and vice-versa
// in a given array
static void replace(int[] arr)
{
     
    // Stores length of array
    int n = arr.Length;
 
    // Traverse array
    for(int i = 0; i < n; i++)
    {
        for(int j = i + 1; j < n; j++)
        {
             
            // If current element is even
            // then swap it with odd
            if (arr[i] >= 0 &&
                arr[j] >= 0 &&
                arr[i] % 2 == 0 &&
                arr[j] % 2 != 0)
            {
                 
                // Perform Swap
                int tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
                 
                // Change the sign
                arr[j] = -arr[j];
 
                break;
            }
 
            // If current element is odd
            // then swap it with even
            else if (arr[i] >= 0 &&
                     arr[j] >= 0 &&
                     arr[i] % 2 != 0 &&
                     arr[j] % 2 == 0)
            {
                 
                // Perform Swap
                int tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
                 
                // Change the sign
                arr[j] = -arr[j];
 
                break;
            }
        }
    }
 
    // Marked element positive
    for(int i = 0; i < n; i++)
        arr[i] = Math.Abs(arr[i]);
 
    // Print readonly array
    for(int i = 0; i < n; i++)
        Console.Write(arr[i] + " ");
}
 
// Driver Code
public static void Main(String[] args)
{
     
    // Given array []arr
    int[] arr = { 1, 3, 2, 4 };
 
    // Function Call
    replace(arr);
}
}
 
// This code is contributed by Rajput-Ji

Javascript

<script>
// Javascript program to implement
// the above approach
 
    // Function to replace each even
    // element by odd and vice-versa
    // in a given array
    function replace(arr)
    {
        // Stores length of array
        let n = arr.length;
  
        // Traverse array
        for (let i = 0; i < n; i++) {
  
            for (let j = i + 1; j < n; j++) {
  
                // If current element is even
                // then swap it with odd
                if (arr[i] >= 0
                    && arr[j] >= 0
                    && arr[i] % 2 == 0
                    && arr[j] % 2 != 0) {
  
                    // Perform Swap
                    let tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
  
                    // Change the sign
                    arr[j] = -arr[j];
  
                    break;
                }
  
                // If current element is odd
                // then swap it with even
                else if (arr[i] >= 0
                         && arr[j] >= 0
                         && arr[i] % 2 != 0
                         && arr[j] % 2 == 0) {
  
                    // Perform Swap
                    let tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
  
                    // Change the sign
                    arr[j] = -arr[j];
  
                    break;
                }
            }
        }
  
        // Marked element positive
        for (let i = 0; i < n; i++)
            arr[i] = Math.abs(arr[i]);
  
        // Prlet final array
        for (let i = 0; i < n; i++)
            document.write(arr[i] + " ");
    }
 
 
    // Driver Code
      
           // Given array arr[]
        let arr = [ 1, 3, 2, 4 ];
  
        // Function Call
        replace(arr);
   
</script>

Output

2 4 1 3

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

Efficient Approach: To optimize the above approach, the idea is to use Two Pointer Approach. Traverse the array by picking each element that is greater than 0 and search for the opposite parity element greater than 0 from the current index up to the end of the array. If found, swap the elements and multiply them with -1. Follow the below steps to solve the problem:

Initialize the variables e and o with -1 that will store the currently found even and odd number respectively that is not yet taken.
Traverse the given array over the range [0, N – 1] and do the following:
- Pick the element arr[i] if it is greater than 0.
- If arr[i] is even, increment o + 1 by 1 and find the next odd number that is not yet marked is found. Mark the current and the found number by multiplying them with -1 and swap them.
- If arr[i] is odd, increment e + 1 by 1 and find the next even number that is not yet marked is found. Mark the current and the found number by multiplying them with -1 and swap them.
After traversing the whole array, multiply its elements with -1 to again make them positive and print that array.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
#define N 3
#define M 4
 
// Function to replace odd elements
// with even elements and vice versa
void swapEvenOdd(int arr[], int n)
{
    int o = -1, e = -1;
 
    // Traverse the given array
    for(int i = 0; i < n; i++)
    {
         
        // If arr[i] is visited
        if (arr[i] < 0)
            continue;
 
        int r = -1;
 
        if (arr[i] % 2 == 0)
        {
            o++;
             
            // Find the next odd element
            while (arr[o] % 2 == 0 || arr[o] < 0)
                o++;
                 
            r = o;
        }
        else
        {
            e++;
             
            // Find next even element
            while (arr[e] % 2 == 1 || arr[e] < 0)
                e++;
                 
            r = e;
        }
 
        // Mark them visited
        arr[i] *= -1;
        arr[r] *= -1;
 
        // Swap them
        int tmp = arr[i];
        arr[i] = arr[r];
        arr[r] = tmp;
    }
 
    // Print the final array
    for(int i = 0; i < n; i++)
    {
        cout << (-1 * arr[i]) << " ";
    }
}
 
// Driver Code
int main()
{
     
    // Given array arr[]
    int arr[] = { 1, 3, 2, 4 };
     
    // Length of the array
    int n = sizeof(arr) / sizeof(arr[0]);
     
    // Function Call
    swapEvenOdd(arr, n);
}
 
// This code is contributed by Rajput-Ji

Java

// Java program for the above approach
 
import java.io.*;
 
class GFG {
 
    // Function to replace odd elements
    // with even elements and vice versa
    static void swapEvenOdd(int arr[])
    {
        // Length of the array
        int n = arr.length;
 
        int o = -1, e = -1;
 
        // Traverse the given array
        for (int i = 0; i < n; i++) {
 
            // If arr[i] is visited
            if (arr[i] < 0)
                continue;
 
            int r = -1;
 
            if (arr[i] % 2 == 0) {
                o++;
 
                // Find the next odd element
                while (arr[o] % 2 == 0
                       || arr[o] < 0)
                    o++;
                r = o;
            }
            else {
                e++;
 
                // Find next even element
                while (arr[e] % 2 == 1
                       || arr[e] < 0)
                    e++;
                r = e;
            }
 
            // Mark them visited
            arr[i] *= -1;
            arr[r] *= -1;
 
            // Swap them
            int tmp = arr[i];
            arr[i] = arr[r];
            arr[r] = tmp;
        }
 
        // Print the final array
        for (int i = 0; i < n; i++) {
            System.out.print(
                (-1 * arr[i]) + " ");
        }
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Given array arr[]
        int arr[] = { 1, 3, 2, 4 };
 
        // Function Call
        swapEvenOdd(arr);
    }
}

Python3

# Python3 program for the above approach
 
# Function to replace odd elements
# with even elements and vice versa
def swapEvenOdd(arr):
     
    # Length of the array
    n = len(arr)
 
    o = -1
    e = -1
 
    # Traverse the given array
    for i in range(n):
         
        # If arr[i] is visited
        if (arr[i] < 0):
            continue
 
        r = -1
 
        if (arr[i] % 2 == 0):
            o += 1
 
            # Find the next odd element
            while (arr[o] % 2 == 0 or
                   arr[o] < 0):
                o += 1
                 
            r = o
        else:
            e += 1
 
            # Find next even element
            while (arr[e] % 2 == 1 or
                   arr[e] < 0):
                e += 1
                 
            r = e
 
        # Mark them visited
        arr[i] *= -1
        arr[r] *= -1
 
        # Swap them
        tmp = arr[i]
        arr[i] = arr[r]
        arr[r] = tmp
 
    # Print the final array
    for i in range(n):
        print((-1 * arr[i]), end = " ")
 
# Driver Code
if __name__ == '__main__':
     
    # Given array arr
    arr = [ 1, 3, 2, 4 ]
 
    # Function Call
    swapEvenOdd(arr)
 
# This code is contributed by Amit Katiyar

C#

// C# program for the above approach
using System;
 
class GFG{
 
// Function to replace odd elements
// with even elements and vice versa
static void swapEvenOdd(int []arr)
{
     
    // Length of the array
    int n = arr.Length;
 
    int o = -1, e = -1;
 
    // Traverse the given array
    for(int i = 0; i < n; i++)
    {
         
        // If arr[i] is visited
        if (arr[i] < 0)
            continue;
 
        int r = -1;
 
        if (arr[i] % 2 == 0)
        {
            o++;
             
            // Find the next odd element
            while (arr[o] % 2 == 0 ||
                   arr[o] < 0)
                o++;
                 
            r = o;
        }
        else
        {
            e++;
 
            // Find next even element
            while (arr[e] % 2 == 1 ||
                   arr[e] < 0)
                e++;
                 
            r = e;
        }
 
        // Mark them visited
        arr[i] *= -1;
        arr[r] *= -1;
 
        // Swap them
        int tmp = arr[i];
        arr[i] = arr[r];
        arr[r] = tmp;
    }
 
    // Print the readonly array
    for(int i = 0; i < n; i++)
    {
        Console.Write((-1 * arr[i]) + " ");
    }
}
 
// Driver Code
public static void Main(String[] args)
{
     
    // Given array []arr
    int []arr = { 1, 3, 2, 4 };
 
    // Function Call
    swapEvenOdd(arr);
}
}
 
// This code is contributed by Amit Katiyar

Javascript

<script>
// JavaScript program for the above approach
 
 
// Function to replace odd elements
// with even elements and vice versa
function swapEvenOdd(arr, n)
{
    let o = -1, e = -1;
 
    // Traverse the given array
    for(let i = 0; i < n; i++)
    {
         
        // If arr[i] is visited
        if (arr[i] < 0)
            continue;
 
        let r = -1;
 
        if (arr[i] % 2 == 0)
        {
            o++;
             
            // Find the next odd element
            while (arr[o] % 2 == 0 || arr[o] < 0)
                o++;
                 
            r = o;
        }
        else
        {
            e++;
             
            // Find next even element
            while (arr[e] % 2 == 1 || arr[e] < 0)
                e++;
                 
            r = e;
        }
 
        // Mark them visited
        arr[i] *= -1;
        arr[r] *= -1;
 
        // Swap them
        let tmp = arr[i];
        arr[i] = arr[r];
        arr[r] = tmp;
    }
 
    // Print the final array
    for(let i = 0; i < n; i++)
    {
        document.write((-1 * arr[i]) + " ");
    }
}
 
// Driver Code
     
    // Given array arr[]
    let arr = [ 1, 3, 2, 4 ];
     
    // Length of the array
    let n = arr.length;
     
    // Function Call
    swapEvenOdd(arr, n);
 
 
//This code is contributed by Mayank Tyagi
</script>

Output

2 4 1 3

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

Efficient Approach 2 :

Another way to do this would be by using a stack. Follow the below steps:

Take element at index from the array arr[] and push to a stack
Iterate arr[] from index i = 1 to end and do the following:
1. If arr[i] and item on top of the stack are not both even or not both odd, pop and swap
2. Else push item to stack

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to replace odd elements
// with even elements and vice versa
void swapEvenOdd(int arr[], int N)
{
  stack<pair<int, int> > stack;
 
  // Push the first element to stack
  stack.push({ 0, arr[0] });
 
  // iterate the array and swap even and odd
  for (int i = 1; i < N; i++)
  {
    if (!stack.empty())
    {
      if (arr[i] % 2 != stack.top().second % 2)
      {
 
        // pop and swap
        pair<int, int> pop = stack.top();
        stack.pop();
 
        int index = pop.first, val = pop.second;
        arr[index] = arr[i];
        arr[i] = val;
      }
      else
        stack.push({ i, arr[i] });
    }
    else
      stack.push({ i, arr[i] });
  }
 
  // print the arr[]
  for (int i = 0; i < N; i++)
    cout << arr[i] << " ";
}
 
// Driven Program
int main()
{
 
  // Given array arr[]
  int arr[] = { 1, 3, 2, 4 };
 
  // Stores the length of array
  int N = sizeof(arr) / sizeof(arr[0]);
 
  // Function Call
  swapEvenOdd(arr, N);
  return 0;
}
 
// This code is contributed by Kingash.

Java

// Java program for the above approach
import java.io.*;
import java.lang.*;
import java.util.*;
 
class GFG {
 
  // Function to replace odd elements
  // with even elements and vice versa
  static void swapEvenOdd(int arr[], int N)
  {
 
    Stack<int[]> stack = new Stack<>();
 
    // Push the first element to stack
    stack.push(new int[] { 0, arr[0] });
 
    // iterate the array and swap even and odd
    for (int i = 1; i < N; i++)
    {
      if (!stack.isEmpty())
      {
        if (arr[i] % 2 != stack.peek()[1] % 2)
        {
 
          // pop and swap
          int pop[] = stack.pop();
          int index = pop[0], val = pop[1];
          arr[index] = arr[i];
          arr[i] = val;
        }
        else
          stack.push(new int[] { i, arr[i] });
      }
      else
        stack.push(new int[] { i, arr[i] });
    }
 
    // print the arr[]
    for (int i = 0; i < N; i++)
      System.out.print(arr[i] + " ");
  }
 
  // Driver code
  public static void main(String[] args)
  {
 
    // Given array arr
    int arr[] = { 1, 3, 2, 4 };
 
    // length of the arr
    int N = arr.length;
 
    // Function Call
    swapEvenOdd(arr, N);
  }
}
 
// This code is contributed by Kingash.

Python3

# Python3 program for the above approach
 
# Function to replace odd elements
# with even elements and vice versa
def swapEvenOdd(arr):
    stack = []
     
    # Push the first element to stack
    stack.append((0, arr[0],))
     
    # iterate the array and swap even and odd
    for i in range(1, len(arr)):
        if stack:
            if arr[i]%2 != stack[-1][1]%2:
                #pop and swap
                index, val = stack.pop(-1)
                arr[index] = arr[i]
                arr[i] = val
            else:
                stack.append((i, arr[i],))
        else:
            stack.append((i, arr[i],))
    return arr
 
# Driver Code
if __name__ == '__main__':
     
    # Given array arr
    arr = [ 1, 3, 2, 4 ]
 
    # Function Call
    print(swapEvenOdd(arr))
 
# This code is contributed by Arunabha Choudhury

Output

[4, 2, 3, 1]

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

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