Question

I’m working on Project Euler problem #2:

Each new term in the Fibonacci sequence is generated
by adding the previous two terms. By
starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …

Find the sum of all the even-valued terms in the sequence which do not exceed four million.

My code:

public class Two {
    public static void main(String[] args) {
        Two obj = new Two();
        int sum = 0, i = 1;

        while (obj.fibonacci(i) < 4000001) {    
            if (obj.fibonacci(i) % 2 == 0) {
                sum += obj.fibonacci(i);
                i++;
            }
        }
        System.out.println(sum);
    }

    public int fibonacci(int n) {
        if (n == 0) {
            return -1;
        }
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 3;
        } else {
            return fibonacci(n - 1) + fibonacci(n - 2);
        }
    }
}

Please help me that what is wrong with this code that when I run it. It doesn’t show the output on the console and the total time will be over than 5 minutes

Thanks

Answer 1

You’re stuck in an infinite loop there as you’re only increasing i when its mod 2 is equal to 0. You need to move your i++ lower.

while (obj.fibonacci(i) <= 4000000) {
    if (obj.fibonacci(i) % 2 == 0) {
        sum += obj.fibonacci(i);
    }
    i++;
}

As other comments have metioned, this isn’t the best way to solve the fibonacci problem, but it solves your error/problem. You should walk this through a debugger if you don’t see why and you’ll notice you use a lot of recursive calls which have already been solved. Since you’re calling it numerous times in the code, (in the while statement and in the if statement) you’ve increased your processing time.

Here is a sample of your fibonacci calls, notice how you call the fibonacci method on the same number multiple times:

1
2
3
2
1
4
3
2
1
2
5