OutOfMemoryException, Result of Loop/While?

Question

I'm working on a project in C#, for which one of the questions is to make a program that receives a Stack and returns a new Stack with the digits that appear in that stack (every digit that appears will be in the new stack only once).

I made this code, which works pretty fine, but in the middle it just stops working and it gives this error "Unhandled Exception: OutOfMemoryException".

I was wondering what was the problem? From what I can understand it occurs because my program requires more memory than my computer can provide. Correct me if I'm wrong.

Does anyone know what I can do to fix this or how I can improve my code to prevent this?

What can I do in general in codes to prevent this problem from occurring in the future?

Thank you very much in advance. :)

public static Stack<int> Stk(Stack<int> S)
    {
        int Num;
        Stack<int> New = new Stack<int>();
        while (!S.IsEmpty())
        {
            Num = S.Pop();
            while (Num != 0)
            {
                if (!Check(New, (Num % 10)))
                {
                    New.Push(Num % 10);
                }
                Console.WriteLine("Original Stack: " + New);
                Num = Num / 10;
            }
        }
        return New;
    }

    public static bool Check(Stack<int> S, int num)
    {
        Stack<int> Temp = new Stack<int>();
        while (!S.IsEmpty())
        {
            Console.WriteLine("Stack Temp: " + Temp);
            if (num == S.Top())
            {
                while (!Temp.IsEmpty())
                {
                    S.Push(Temp.Top());
                }
                Console.WriteLine("Number found in Stack S!");
                Console.WriteLine("Stack S: " + S);
                return true;
            }
            Temp.Push(S.Pop());
        }
        while (!Temp.IsEmpty())
        {
            S.Push(Temp.Pop());
        }
        Console.WriteLine("Stack S: " + S);
        Console.WriteLine("Number NOT found in Stack S!");
        return false;
    }

Answer 1

if (num == S.Top())
{
    while (!Temp.IsEmpty()) // Infinite loop, Temp is never empty
    {
        S.Push(Temp.Top()); // Because of infinite loop, S is just going to fill up memory
    }

    //...

I'm not familiar with Top() on a Stack<T> but judging from the rest of the code it looks like it doesn't remove an object from your stack (the function I know of that does this is Peek). If that's the case you've got an infinite loop that fills up all available memory and ultimately runs out.

Edit: This answer focuses on the problem you're having. This smells like homework to me so I'm not going to rewrite it but I'd suggest paying at @ckuri's comment on this answer down below. It looks like you're doing a lot more work than you should. You may want to look at iterating the stack rather than popping to a whole new stack.