Arrange 0's & 1's in a array

Question

This is one of an interview question which I had recently. I would like to know others perception of approach for this problem.

Question:

Yo

Answer 1

Okey, here is my approach.

e.g a[] = { 1,0,0,0,1,1,1,0,0,1};

Pseudocode:

1. Have two counters, count1 = 0 and count2 = (n/2)+1

2. Traverse through the array,

   if(arr[ i ] == 1) 
   { 
       arr[ i ] = count1++;
   } else { 
       arr[ i ] = count2++ 
   };
   

3. At the end of the traversal, you have array filled with numbers 0 to n-1 like:

   a[ ] = { 0, 5, 6, 7, 1, 2, 3, 8, 9 4}
   

4. Now the problem comes to sort the above resultant array, this can be done in O(N) as below:

   for(j = 0; j <= 1; j++)  
   {
       for(i = 0; i

   Note: j loop runs only twice irrespective on 'n' and has constant complexity. The order of this whole loop is 2*n = O(n).

5. After the array is sorted, Again traverse through the array and set elements arr[0] to arr[n/2] to '1' and arr[(n/2)+1] to arr[n] as '0'.

Space complexity is constant and time complexity is O(step2) + O(step4) + O(step5) = n + 2n +n = 4*n = O(n).