Question 21 : Separate odd and even numbers in an array

 Given an array of integers, you need to segregate odd and even numbers in an array.

Please note: Order of elements can be changed.

For example:

arr[] = {12, 17, 70, 15, 22, 65, 21, 90}
Array after separating odd and even numbers :
{12, 90, 70, 22, 15, 65, 21, 17}

• Algorithm

• Let's say array is arr[]
• Initialise two index variable , left=0 and right=arr.length-1
• Increment left variable until you get odd number
• Decrement right variable until you get even number.
• If left < right, swap arr[left] and arr[right]
• In the end, you will see that you have even numbers on left side and odd numbers on right side.

Java code to separate odd and even numbers in an array 

public class SeparateOddEvenMain {

    public static void main(String[] args) {

        int arr[] = {
            12,
            17,
            70,
            15,
            22,
            65,
            21,
            90
        };

        System.out.println("Original Array: ");
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }

        arr = separateEvenOddNumbers(arr);
        System.out.println("nArray after separating even and odd numbers : ");
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }

    }

    public static int[] separateEvenOddNumbers(int arr[]) {
        int left = 0;
        int right = arr.length - 1;

        for (int i = 0; i < arr.length; i++) {

            while (arr[left] % 2 == 0) {
                left++;
            }
            while (arr[right] % 2 == 1) {
                right--;
            }

            if (left < right) {
                int temp = arr[left];
                arr[left] = arr[right];
                arr[right] = temp;
            }
        }

        return arr;
    }
}

when you run the above program, you will get the below output:

Original Array: 
12 17 70 15 22 65 21 90 

Array after separating even and odd numbers : 
12 90 70 22 15 65 21 17

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