Task:
Separate even and odd numbers from an array of integers. Put all even numbers first, and then odd numbers.
Given:
An array having even and odd integers. We have to segregate even and odd integers and put even numbers first and then odd numbers.
Example:

Input: {2, 5, 1, 10, 3, 6, 7, 8}

Output: 2 8 6 10 3 1 7 5

# Algorithm
Initialise two index variables left = 0 and right = n-1.

We have to increment the left index until we see an odd number.

We have to decrement the right index until we see an even number.

If left < right then swap array[left] and array[right].

Code:

#include
using namespace std;

void swap(int * x, int * y){
  int t = * x;
  * x = * y;
  * y = t;
}

void segregate(int array[], int n){
  int left = 0, right = n - 1;
  while (left < right){
    while (array[left] % 2 == 0 && left < right)
      left++;
    while (array[right] % 2 == 1 && left < right)
      right--;
    if (left < right){
      swap( & array[left], & array[right]);
      left++;
      right--;
    }
  }
}

int main(){

  int array[100], n, i;
  cout << "Enter number of elements: ";
  cin >> n;
  cout << "\nEnter elements: ";

  for (i = 0; i < n; i++)
    cin >> array[i];

  cout << "Original array: ";
  for (int i = 0; i < n; i++)
    cout << array[i] << " ";

  segregate(array, n);
  cout << "\nArray after divided: ";

  for (int i = 0; i < n; i++)
    cout << array[i] << " ";

  return 0;

}