CollatzSets

import java.util.ArrayList;
import java.util.Collections;

public class CollatzSets {
    public static ArrayList<Integer> genCollatzSequence(int n) {
        ArrayList<Integer> sequence = new ArrayList<>();
        while (n != 1) {
            sequence.add(n);
            if (n % 2 == 0) {
                n /= 2;
            } else {
                n = n * 3 + 1;
            }
        }
        sequence.add(1);
        return sequence;
    }

    public static ArrayList<Integer> getIntersectionOfSets(
            ArrayList<Integer> A, ArrayList<Integer> B) {
        ArrayList<Integer> intersection = new ArrayList<>();
        for (int i = 0; i < A.size(); i++) {
            int elem = A.get(i);
            if (B.indexOf(elem) >= 0) {
                intersection.add(elem);
            }
        }
        Collections.sort(intersection);
        return intersection;
    }

    public static ArrayList<Integer> getUnionOfSets(ArrayList<Integer> A,
            ArrayList<Integer> B) {
        ArrayList<Integer> union = getIntersectionOfSets(A, B);
        for (int i = 0; i < A.size(); i++) {
            int elem = A.get(i);
            if (B.indexOf(elem) < 0) {
                union.add(elem);
            }
        }
        for (int i = 0; i < B.size(); i++) {
            int elem = B.get(i);
            if (A.indexOf(elem) < 0) {
                union.add(elem);
            }
        }
        Collections.sort(union);
        return union;
    }

    public static ArrayList<Integer> getUnionOfCollatz(int a, int b) {
        return getUnionOfSets(genCollatzSequence(a), genCollatzSequence(b));
    }

    public static ArrayList<Integer> getIntersectionOfCollatz(int a, int b) {
        return getIntersectionOfSets(genCollatzSequence(a), genCollatzSequence(b));
    }
}