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I am trying to do a simple coding challenge that requires me to do the following:

You are given n, return an array ans, composed in such way:

`ans = [n, n - 5, n - 10, ... , m, m + 5, ... , n - 5, n]`, where m stands for the first non-positive integer obtained by subtractions.

Try to solve it without any loop.

Example

For n = 25, the output should be

listWithoutLoop(n) = [25, 20, 15, 10, 5, 0, 5, 10, 15, 20, 25].

I have done this code:

int[] listWithoutLoop(int n)
{
    List<int> test = new List<int>();

    if (test.Count > 2 && test[test.Count - 1] == n)
        return test.ToArray();

    if (n <= 0)
    {
        test.Add(n + 5);
        return listWithoutLoop(n + 5);
    }
    else
    {
        test.Add(n - 5);
        return listWithoutLoop(n - 5);
    }
}

But I keep getting a stack overflow when running it. Is recursion supported by c#? If so, how to prevent getting a stackoverflow exception when running it?

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5  
You keep re-creating test in every recursive call so test.Count > 2 is always false so you never reach the base case. – Lee Dec 14 '17 at 12:53
1  
StackOverflow in recursion is always because of the same thing: your exit condition is never reached. – Pikoh Dec 14 '17 at 12:53
    
What happens if you move List<int> test = new List<int>(); above int[] listWithoutLoop(int n)? – mjwills Dec 14 '17 at 12:54
    
For the last part : of course C# supports recursion. Did you try your code in any other language ? It should raise the same error. – Pac0 Dec 14 '17 at 12:55
3  
Make list argument of your function and pass it to every recursive call. – Evk Dec 14 '17 at 12:59
up vote -1 down vote accepted

To simplify, I splitted the function to add up and add down separately (it's always better to have simple and understandable code at all times)

    static void Main()
    {
        int n = 20;
        int interval = 5;

        List<int> list = new List<int>();
        AddDown(list, n, 0, interval);
        AddUp(list, 0, n, interval);

        int[] arrInt = list.ToArray();

    }

    static void AddDown(List<int> list, int currentNumber, int targetNumber, int interval)
    {
        if(currentNumber > targetNumber)
        {
            list.Add(currentNumber);
            AddDown(list, currentNumber - interval, targetNumber, interval);
        }
    }

    static void AddUp(List<int> list, int currentNumber, int targetNumber, int interval)
    {
        if (currentNumber <= targetNumber)
        {
            list.Add(currentNumber);
            AddUp(list, currentNumber + interval, targetNumber, interval);
        }
    }
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up vote 1 down vote

You must define the test list above the listWithoutLoop() method.

List<int> test = new List<int>();
int[] listWithoutLoop(int n)
{
....
}
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