Question 20: Given an array of 0’s and 1’s in random order, you need to separate 0’s and 1’s in an array

For example::

arr[] = {0,1,0,0,1,1,1,0,1}
Array after separating 0 and 1 numbers :
{0,0,0,0,1,1,1,1,1}

Solution :

Solution 1:

• Count number of 0’s in the array. Lets say we get X 0’s
• Once we get the count, put X 0’s in the array and put (n-X) 1’s in the latter part of array.

Java code:

public static int[] separate0s1sSolution1(int arr[])
 {
  int count=0;
  for (int i = 0; i < arr.length; i++) {
   if(arr[i]==0)
   {
    count++;
   }
  }
  for (int i = 0; i < count; i++) {
   arr[i]=0;
  }
  for (int i = count; i < arr.length; i++) {
   arr[i]=1;
  }
  return arr;
 }

Time Complexity : O(N)

Solution 2:

Algorithm: 

Lets say array is arr[]

• Initialise two index variable , left=0 and right=arr.length-1
• Increment left variable until you get 1’s.
• Decrement right variable until you get 0’s
• If left < right, swap arr[left] and arr[right]
• In the end, you will see that you have 0’s on left side and 1’s on right side.

Java code:

public static int[] separate0s1sSolution2(int arr[])
 {
  for (int i = 0; i < arr.length; i++) {
   int left=0;
   int right=arr.length-1;
   while(arr[left]==0)
   {
    left++;
   }
   while(arr[right]==1)
   {
    right--;
   }
 
   if(left<right)
   {
    int temp=arr[left];
    arr[left]=arr[right];
    arr[right]=temp;
   }
  }
  return arr;
 }

Java code to separate odd and even numbers in an array 

public class Separate0s1sMain {

    public static void main(String[] args) {
        int arr[] = {
            0,
            1,
            0,
            0,
            1,
            1,
            1,
            0,
            1
        };

        System.out.println("Original Array: ");

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

        arr = separate0s1sSolution1(arr);
        System.out.println("n===========================");
        System.out.println("Solution 1");
        System.out.println("nArray after separating 0's and 1's : ");

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

        System.out.println("n===========================");
        System.out.println("Solution 2");

        arr = separate0s1sSolution2(arr);
        System.out.println("nArray after separating 0's and 1's : ");
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }

    public static int[] separate0s1sSolution1(int arr[]) {
        int count = 0;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == 0) {
                count++;
            }
        }
        for (int i = 0; i < count; i++) {
            arr[i] = 0;
        }
        for (int i = count; i < arr.length; i++) {
            arr[i] = 1;
        }
        return arr;
    }

    public static int[] separate0s1sSolution2(int arr[]) {
        for (int i = 0; i < arr.length; i++) {
            int left = 0;
            int right = arr.length - 1;
            while (arr[left] == 0) {
                left++;
            }
            while (arr[right] == 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: 
0 1 0 0 1 1 1 0 1 
===========================
Solution 1
 
Array after separating 0's and 1's : 
0 0 0 0 1 1 1 1 1 
===========================
Solution 2
 
Array after separating 0's and 1's : 
0 0 0 0 1 1 1 1 1

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