Even numbers at even index and odd numbers at odd index

Given an array of size n containing equal number of odd and even numbers. The problem is to arrange the numbers in such a way that all the even numbers get the even index and odd numbers get the odd index. Required auxiliary space is O(1).

Examples :

Input : arr[] = {3, 6, 12, 1, 5, 8}
Output : 6 3 12 1 8 5 

Input : arr[] = {10, 9, 7, 18, 13, 19, 4, 20, 21, 14}
Output : 10 9 18 7 20 19 4 13 14 21

Source: Amazon Interview Experience | Set 410.

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach :

Start from the left and keep two index one for even position and other for odd positions.
Traverse these index from left.
At even position there should be even number and at odd positions, there should be odd number.
Whenever there is mismatch , we swap the values at odd and even index.

Below is the implementation of the above approach :

CPP

// C++ implementation to arrange 
// odd and even numbers 
#include <bits/stdc++.h> 
using namespace std; 
  
// function to arrange odd and even numbers 
void arrangeOddAndEven(int arr[], int n) 
{ 
   int oddInd = 1; 
    int evenInd = 0; 
    while (true) 
    { 
        while (evenInd < n && arr[evenInd] % 2 == 0) 
            evenInd += 2; 
              
        while (oddInd < n && arr[oddInd] % 2 == 1) 
            oddInd += 2; 
              
        if (evenInd < n && oddInd < n) 
            swap (arr[evenInd], arr[oddInd]); 
              
        else
            break; 
    } 
} 
  
// function to print the array 
void printArray(int arr[], int n) 
{ 
    for (int i = 0; i < n; i++) 
        cout << arr[i] << " "; 
} 
  
// Driver program to test above 
int main() 
{ 
    int arr[] = { 3, 6, 12, 1, 5, 8 }; 
    int n = sizeof(arr) / sizeof(arr[0]); 
  
    cout << "Original Array: "; 
    printArray(arr, n); 
  
    arrangeOddAndEven(arr, n); 
  
    cout << "\nModified Array: "; 
    printArray(arr, n); 
  
    return 0; 
} 

Java

// Java implementation to 
// arrange odd and even numbers 
  
import java.util.*; 
import java.lang.*; 
  
class GfG { 
  
// function to arrange 
// odd and even numbers 
public static void arrangeOddAndEven(int arr[], int n) 
{ 
    int oddInd = 1; 
    int evenInd = 0; 
    while (true) 
    { 
        while (evenInd < n && arr[evenInd] % 2 == 0) 
            evenInd += 2; 
              
        while (oddInd < n && arr[oddInd] % 2 == 1) 
            oddInd += 2; 
              
        if (evenInd < n && oddInd < n) 
            { 
                int temp = arr[evenInd]; 
                arr[evenInd] = arr[oddInd]; 
                arr[oddInd] = temp; 
            } 
              
        else
            break; 
    } 
} 
  
// function to print the array 
public static void printArray(int arr[], int n) 
{ 
    for (int i = 0; i < n; i++) 
        System.out.print(arr[i] + " "); 
} 
      
// Driver function  
public static void main(String argc[]){ 
    int arr[] = { 3, 6, 12, 1, 5, 8 }; 
    int n = 6; 
  
    System.out.print("Original Array: "); 
    printArray(arr, n); 
  
    arrangeOddAndEven(arr, n); 
  
    System.out.print("\nModified Array: "); 
    printArray(arr, n); 
} 
} 
  
// This code is contributed by Sagar Shukla  

Python3

      
# Python3 implementation to 
# arrange odd and even numbers 
  
def arrangeOddAndEven(arr,  n): 
      
    oddInd = 1
    evenInd = 0
    while (True): 
          
        while (evenInd < n and arr[evenInd] % 2 == 0): 
            evenInd += 2
               
        while (oddInd < n and arr[oddInd] % 2 == 1): 
            oddInd += 2
               
        if (evenInd < n and oddInd < n): 
                temp = arr[evenInd] 
                arr[evenInd] = arr[oddInd] 
                arr[oddInd] = temp; 
               
        else: 
            break
   
# function to print the array 
def printArray(arr,  n): 
    for i in range(0,n): 
        print(arr[i] , "",end="") 
  
       
# Driver function  
def main(): 
    arr = [ 3, 6, 12, 1, 5, 8 ] 
    n = 6
   
    print("Original Array: ",end="") 
    printArray(arr, n) 
   
    arrangeOddAndEven(arr, n) 
   
    print("\nModified Array: ",end="") 
    printArray(arr, n) 
      
if __name__ == '__main__': 
    main() 
# This code is contributed by 29AjayKumar 

C#

// C# implementation to 
// arrange odd and even numbers 
using System; 
  
class GFG { 
  
    // function to arrange 
    // odd and even numbers 
    public static void arrangeOddAndEven(int[] arr, int n) 
    { 
     int oddInd = 1; 
     int evenInd = 0; 
    while (true) 
    { 
        while (evenInd < n && arr[evenInd] % 2 == 0) 
            evenInd += 2; 
               
        while (oddInd < n && arr[oddInd] % 2 == 1) 
            oddInd += 2; 
               
        if (evenInd < n && oddInd < n) 
            { 
                int temp = arr[evenInd]; 
                arr[evenInd] = arr[oddInd]; 
                arr[oddInd] = temp; 
            } 
               
        else
            break; 
    } 
    } 
  
    // function to print the array 
    public static void printArray(int[] arr, int n) 
    { 
        for (int i = 0; i < n; i++) 
            Console.Write(arr[i] + " "); 
    } 
  
    // Driver function 
    public static void Main() 
    { 
        int[] arr = { 3, 6, 12, 1, 5, 8 }; 
        int n = 6; 
  
        Console.Write("Original Array: "); 
        printArray(arr, n); 
  
        arrangeOddAndEven(arr, n); 
  
        Console.Write("\nModified Array: "); 
        printArray(arr, n); 
    } 
} 
  
// This code is contributed by Sam007 

Output :

Original Array: 3 6 12 1 5 8 
Modified Array: 6 3 12 1 8 5

Time Complexity: O(n).
Auxiliary Space: O(1).

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