Segregate even and odd numbers using Lomuto’s Partition Scheme

Given an array arr[] of integers, segregate even and odd numbers in the array such that all the even numbers should be present first, and then the odd numbers.

Examples: 

Input: arr[] = {7, 2, 9, 4, 6, 1, 3, 8, 5}
Output: 2 4 6 8 7 9 1 3 5

Input: arr[] = {1, 3, 2, 4, 7, 6, 9, 10}
Output:  2 4 6 10 7 1 9 3

We have discussed two different approaches in below posts: 

• Segregate Even and Odd numbers
• Segregate even and odd numbers | Set 2

Approach: The optimization for the above approach is based on Lomuto’s Partition Scheme

• Start with an array of numbers.

For the sake of illustration, let’s consider the following array: [7, 2, 9, 4, 6, 1, 3, 8, 5].

• Choose a pivot element from the array.

In this case, let’s choose the last element, which is 5, as the pivot.

• Initialize two pointers, i and j.
  • The pointer i will keep track of the boundary between the even and odd numbers, and
  • the pointer j will iterate through the array.
• Iterate through the array using the pointer j.
  • Compare each element with the pivot and perform the following steps:
    • If the element is even, swap it with the element at index i and increment i.
    • If the element is odd, do nothing and continue.

Let’s go through the steps:

• Initially, i = 0 and j = 0.
• The first element is 7 (odd),
  • so we do nothing and move j to the next element.
• The second element is 2 (even),
  • so we swap it with the element at index i (which is also 2) and increment i to 1.
• The third element is 9 (odd),
  • so we do nothing and move j to the next element.
• The fourth element is 4 (even),
  • so we swap it with the element at index i (which is 9) and increment i to 2.
• The fifth element is 6 (even),
  • so we swap it with the element at index i (which is 9) and increment i to 3.
• The sixth element is 1 (odd),
  • so we do nothing and move j to the next element.
• The seventh element is 3 (odd),
  • so we do nothing and move j to the next element.
• The eighth element is 8 (even),
  • so we swap it with the element at index i (which is 1) and increment i to 4.
• The ninth element is 5 (odd),
  • so we do nothing and move j to the next element.

• After iterating through the array, we partitioned the even and odd numbers.

The array now looks like this: [2, 4, 6, 8, 5, 1, 3, 9, 7].

• Finally, swap the pivot element with the element at index I.

In this case, we swap 5 with 7. This step ensures that the pivot element is in its final sorted position.

The array after the final swap will be: [2, 4, 6, 8, 7, 1, 3, 9, 5].

• Now, the even numbers are on the left side of the pivot, and the odd numbers are on the right side. The pivot element is in its correct sorted position.

Below is the implementation of the above approach: 

C++

// CPP code to segregate even odd
// numbers in an array
#include <bits/stdc++.h>
using namespace std;

// Function to segregate even odd numbers
vector<int> arrayEvenAndOdd(vector<int>& arr)
{

    int i = -1, j = 0;
    int pivot = arr.back();
    for (j = 0; j < arr.size() - 1; j++) {
        if (arr[j] % 2 == 0) {
            i++;

            // Swapping even and odd numbers
            swap(arr[i], arr[j]);
        }
    }

    swap(arr[i + 1], arr.back());
    return arr;
}

// Driver code
int main()
{
    vector<int> arr = { 7, 2, 9, 4, 6, 1, 3, 8, 5 };
    vector<int> updatedArr = arrayEvenAndOdd(arr);

    // Function Call
    cout << "Updated Array: ";
    for (int num : updatedArr) {
        cout << num << " ";
    }
    cout << endl;

    return 0;
}

Java

// java code to segregate even odd
// numbers in an array
public class GFG {

    // Function to segregate even
    // odd numbers
    static void arrayEvenAndOdd(int arr[], int n)
    {

        int i = -1, j = 0;
        while (j != n) {
            if (arr[j] % 2 == 0) {
                i++;

                // Swapping even and
                // odd numbers
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
            j++;
        }

        // Printing segregated array
        for (int k = 0; k < n; k++)
            System.out.print(arr[k] + " ");
    }

    // Driver code
    public static void main(String args[])
    {
        int arr[] = { 1, 3, 2, 4, 7, 6, 9, 10 };
        int n = arr.length;
        arrayEvenAndOdd(arr, n);
    }
}

// This code is contributed by Sam007

Python3

# Python3 code to segregate even odd
# numbers in an array

# Function to segregate even odd numbers


def arrayEvenAndOdd(arr, n):
    i = -1
    j = 0
    while (j != n):
        if (arr[j] % 2 == 0):
            i = i+1

            # Swapping even and odd numbers
            arr[i], arr[j] = arr[j], arr[i]

        j = j+1

    # Printing segregated array
    for i in arr:
        print(str(i) + " ", end='')


# Driver Code
if __name__ == '__main__':
    arr = [1, 3, 2, 4, 7, 6, 9, 10]
    n = len(arr)
    arrayEvenAndOdd(arr, n)

# This code was contributed by
# Yatin Gupta

C#

// C# code to segregate even odd
// numbers in an array
using System;

class GFG {

    // Function to segregate even
    // odd numbers
    static void arrayEvenAndOdd(int[] arr, int n)
    {

        int i = -1, j = 0;
        while (j != n) {
            if (arr[j] % 2 == 0) {
                i++;

                // Swapping even and
                // odd numbers
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
            j++;
        }

        // Printing segregated array
        for (int k = 0; k < n; k++)
            Console.Write(arr[k] + " ");
    }

    // Driver code
    static void Main()
    {
        int[] arr = { 1, 3, 2, 4, 7, 6, 9, 10 };
        int n = arr.Length;
        arrayEvenAndOdd(arr, n);
    }
}

// This code is contributed by Sam007

Javascript

<script>
// JavaScript code to segregate even odd
// numbers in an array

// Function to segregate even odd numbers
function arrayEvenAndOdd(arr, n)
{

    let i = -1, j = 0;
    let t;
    while (j != n) {
        if (arr[j] % 2 == 0) {
            i++;

            // Swapping even and odd numbers
                let temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
        }
        j++;
    }

    // Printing segregated array
    for (let i = 0; i < n; i++)
        document.write(arr[i] + " ");
}

// Driver code

    let arr = [ 1, 3, 2, 4, 7, 6, 9, 10 ];
    let n = arr.length;
    arrayEvenAndOdd(arr, n);

// This code is contributed by Surbhi Tyagi

</script>

PHP

<?php
// PHP code to segregate even odd
// numbers in an array

// Function to segregate
// even odd numbers
function arrayEvenAndOdd($arr, $n)
{
    $i = -1;
    $j = 0;
    $t;
    while ($j != $n)
    {
        if ($arr[$j] % 2 == 0)
        {
            $i++;

            // Swapping even and
            // odd numbers
            $x = $arr[$i];
            $arr[$i] = $arr[$j];
            $arr[$j] = $x;
        }
        $j++;
    }

    // Printing segregated 
    // array
    for ($i = 0; $i < $n; $i++)
        echo $arr[$i] . " ";
}

    // Driver code
    $arr = array(1, 3, 2, 4, 7, 6, 9, 10);
    $n = sizeof($arr);
    arrayEvenAndOdd($arr, $n);

// This code is contributed by Anuj_67
?>

Output

Updated Array: 2 4 6 8 5 1 3 7 9 

Time Complexity: O(n) 
Auxiliary Space: O(1), since no extra space has been taken.