The array is already sorted, we can keep two pointers ii and jj, where ii is the slow-runner while jj is the fast-runner. As long as nums[i] = nums[j]nums[i]=nums[j], we increment jj to skip the duplicate.

When we encounter nums[j] != nums[i] the duplicate run has ended so we must copy its value to nums[i + 1]nums[i+1]. ii is then incremented and we repeat the same process again until jj reaches the end of array.

using System;
namespace CsharpCode_ConsoleApplication{
   public class Arrays{
      public int RemoveDuplicatesFromSortedArrayAndReturnLength(int[] arr){
         int index = 1;
         for (int i = 0; i < arr.Length - 1; i++){
            if (arr[i] != arr[i + 1]){
               arr[index] = arr[i + 1];
               index++;
            }
            else{
               continue;
            }
         }
         return index;
      }
   }
   class CsharpCode_Program{
      static void Main(string[] args){
         Arrays a = new Arrays();
         int[] arr = { 0, 0, 1, 1, 1, 2, 2, 3, 3, 4 };
         int res = a.RemoveDuplicatesFromSortedArrayAndReturnLength(arr);
         Console.WriteLine(res);
         Console.ReadLine();
      }
   }
}