How to sort 0,1,2 in an Array (Dutch National Flag) without extra space using C#?

We need to take three-pointers, low, mid, high. We will use low and mid pointers at the start, and the high pointer will point at the end of the given array.

If array [mid] =0, then swap array [mid] with array [low] and increment both pointers once.

If array [mid] = 1, then no swapping is required. Increment mid pointer once.

If array [mid] = 2, then we swap array [mid] with array [high] and decrement the high pointer once.

Time complexity − O(N)

Example

 Live Demo

using System;
namespace ConsoleApplication{
   public class Arrays{
      private void Swap(int[] arr, int pos1, int pos2){
         int temp = arr[pos1];
         arr[pos1] = arr[pos2];
         arr[pos2] = temp;
      }
      public void DutchNationalFlag(int[] arr){
         int low = 0;
         int mid = 0;
         int high = arr.Length - 1;
         while (mid <= high){
            if (arr[mid] == 0){
               Swap(arr, low, mid);
               low++;
               mid++;
            }
            else if (arr[mid] == 2){
               Swap(arr, high, mid);
               high--;
            }
            else{
               mid++;
            }
         }
      }
}
class Program{
   static void Main(string[] args){
         Arrays a = new Arrays();
         int[] arr = { 2, 1, 1, 0, 1, 2, 1, 2, 0, 0, 1 };
         a.DutchNationalFlag(arr);
         for (int i = 0; i < arr.Length; i++){
            Console.WriteLine(arr[i]);
         }
         Console.ReadLine();
      }
   }
}

Output

0 0 0 0 1 1 1 1 2 2 2