[Amazon] Sort array which contains only 0, 1 or 2 in O(n)

Problem: Sort an array which contains only 0, 1 or 2. The time complexity should not be more than O(n).

Example:

Input: arr = [0, 1, 2, 0, 1, 1, 2]
Output: [0, 0, 1, 1, 1, 2, 2]

Approach: We will use three pointer approach here.

Implementation in C#:

    public void SortColors(int[] nums)

    {

        int length = nums?.Length ?? 0;

        if (length <= 1)

        {

            return;

        }

        int low = 0;

        int high = length - 1;

        for (int mid = 0; mid <= high;)

        {

            Console.WriteLine($"{mid}: {nums[mid]}");

            switch(nums[mid])

            {

                case 0: this.Swap(nums, low++, mid++);

                    break;

                case 1: mid++;

                    break;

                case 2: this.Swap(nums, mid, high--);

                    break;

            }

        }

    }

    private void Swap(int[] nums, int i, int j)

    {

        int temp = nums[i];

        nums[i] = nums[j];

        nums[j] = temp;

    }

Complexity: O(n)