Project-Euler / 14

Problem 14 asks:

The following iterative sequence is defined for the set of positive integers:

n → n/2 (n is even)
n → 3n + 1 (n is odd)

Which starting number, under one million, produces the longest chain?

Here’s my solution in Java:

public class Problem14 {
    public static void main(String[] args) {
        final int MAX = 1_000_000;
        int longestChain = 0;
        int result = -1;
        int cache[] = new int[MAX];
        for (int n = 2; n < MAX; n++) {
            int terms = 1;
            long m = n;
            while (m >= n) {
                m = (m % 2 == 0) ? m / 2 : 3 * m + 1;
                terms++;
            }
            terms += cache[(int)m];  // safe to cast because m < n
            cache[n] = terms;
            if (terms > longestChain) {
                longestChain = terms;
                result = n;
            }
        }
        System.out.println(result);
    }
}

It stores the results it finds as it goes, so it doesn’t have to repeat the same computations.

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