Separate 0’s and 1’s in an array.

Question: Given an array arr[], that contains only 0’s and 1’s. Write a function that separate 0’s and 1’s in the array.

Input: 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1
Output: 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1

The objective of the problem is to segregate the array in two parts, and it is more popularly known as a variation of Dutch National Flag Problem.

Method 1:

• The most naive method is to count the number of elements in the given array. Let it be N.
• Do a second scan of the array and count the number of zeroes. Let it be c.
• Now fill the array with c number of 0’s and fill the remaining positions with 1.

Time Complexity:- O(2n)

Method 2:

We can solve this problem in a fashion similar to separating odd and even numbers in an array.

• Initialize 2 indexes, left = 0 and right = size-1.
• Increment left, till it is 0.
• Decrement right till it is 1.
• If (left < right) swap the values at arr[left] and arr[right].

void swap(int *i, int *j)
{
	int temp = *i;
	*i = *j;
	*j = temp;
}

int seperateZeroAndOne(int arr[], int size)
{
	// Initialize left and right indexes
	int left = 0;
	int right = size - 1;
	
	while(left < right)
	{
		// Increment left index till the number is 0
		while(arr[left] == 0 && left < right)
			left++;
		
		// Decrement right index till the number is 1
		while(arr[right] == 1 && left < right)
			right--;
		
		// Time to swap
		if(left < right)
		{
			swap(&arr[left], &arr[right]);
			left++;
			right--;
		}
	}
}

Time Complexity:- O(n)