# Separate 0’s and 1’s in a given array

**Objective**: Given an array which contains only 0’s and 1’s. write an algorithm to separate 0’s and 1’s.

**Example**

int[] arrA = {1,0,1,0,1,1,0,0,0,0,1}; Output: [0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1]

**Approach: Counting**

- Count number of 0’s in the array. Say its count.
- Now in array, put 0 in indexes from 0 to count – 1.
- In rest of the array put 1.

Time Complexity: O(n)

**Code**:

Output: [0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1]

**Approach: Swapping Indexes**

- Use two indexes, left and right.
- Put left index at the start of array and right at the end of the array.
- Increment left till 1 is not found.
- Decrement right till 0 is not found.
- Swap left and right elements
- Do it till left<right

Time Complexity: O(n)

**Code**:

Output:

[0, 0, 0, 1, 1, 1, 1, 1]