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

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 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);        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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Improved By : 29AjayKumar

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