Find duplicate Numbers repeated more than once C#

Question

How do we find duplicates number in int array repeated more than two times. One of the example I have is as below

public static void main()
{
      int[] array = { 3, 7, 9 , 7, 4, 9, 7 , 9}
      Dictionary<int,int> dic = new Dictionary<int,int>();
      foreach(var Val in array)
      {
         if(dict.containskey(Val)
           dic[Val] ++;
         else 
           dic[Val] = 1 ;
      }

   foreach (var pair in dic)
      if(key.value > 2 )
         Console.WriteLine(pair.key)
}

Is there any better way to do this? I was been asked in one of the interview. I gave above solution which was been rejected.

Edit :: I was been asked to write a program which takes less memory and good in performance.. It should not use linq or any inbuilt C# extension method...

Answer 1

Group your items and take only those with more than 2 occurrences:

array.GroupBy(x=>x).Where(x=>x.Count()>2).Select(x=>x.Key)

Answer 2

Since there aren't any constraints provided to the elements that can be contained in this array, you should have asked the interviewer whether he wants a solution with O(n) time complexity and O(n) space complexity** or a solution with O(nlogn) time complexity and O(1) space complexity**.

Without constraints to the elements in the array there's no solution in O(n) time complexity and O(1) space complexity**.

And because he rejected your solution (which is O(n) time complexity and O(n) space complexity**) apparently he was seeking for the second. One way to achieve that is to first sort the array and then iterate over it to find the duplicates.

Remark**: the example values provided for space complexity do not include the space occupied by the original array, only the extra space needed.