How to avoid stackoverflow at collatz for high values?

Asked Sep 11 '15 at 19:48

Active Sep 11 '15 at 20:09

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I'm working on an eulers problem, where you have to get the longest collatz chain. The problem is, you have to find it in a sequence from 1 to 1 000 000. My code works great up to 100 000, then it throws StackOverFlowError. Can I avoid it, or is my code crap?

public class Collatz {
    private static final ArrayList<Integer> previous = new ArrayList<>();
    public static void main(String[] args) {

        for (int i = 1; i < 100000; i++){
            collatzLength(i, 1);
        }
        Integer i = Collections.max(previous);
        System.out.println(i);

    }

    public static void collatzLength(int n, int count){

        if (n == 1){
            previous.add(count);
        } else if (n%2 == 0){
             collatzLength(n/2, count+1);

        } else {
            collatzLength(3*n+1, count+1);
        }
    }
}

edited Sep 11 '15 at 20:09

Luiggi Mendoza

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asked Sep 11 '15 at 19:48

Rok Dolinar

1

Eliminate recursion, or make the stack bigger. – D. Ben Knoble Sep 11 '15 at 19:55
1

Or, use a stack object instead of a virtual memory stack – D. Ben Knoble Sep 11 '15 at 19:56
Do not make your stack bigger. Eliminate recursion. Making the stack bigger is only fighting the symptoms of your bad code. – Florian Schaetz Sep 11 '15 at 20:03
But the iterative version will take 1 year to calculate? – Rok Dolinar Sep 11 '15 at 20:05
If you want to keep with the recursive method, use memoization to store the results of previous calculations, that will speed up the algorithm and lower the number of methods in the stack. – Luiggi Mendoza Sep 11 '15 at 20:08
@RokDolinar: What made you think that? You're not doing anything in the recursive version that would be different or perform differently in the iterative version. – Louis Wasserman Sep 11 '15 at 20:10
Alright nvm, I did the iterative version, but it "lags" at 113382. Why? – Rok Dolinar Sep 11 '15 at 20:19
OHHHHH size of int is not sufficient – Rok Dolinar Sep 11 '15 at 20:26
Recursion is normally not faster than iterative approaches. Most of the time you can find a much less memory and cpu intensive iterative solution. – Florian Schaetz Sep 12 '15 at 08:26

How to avoid stackoverflow at collatz for high values?

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