Karatsuba DC

    private static int nDigits(long x) {
        int digits = 0;
        do {
            digits++;
            x /= 10;
        } while (x != 0);
        return digits;
    }

    private static long multiply(long x, long y) {
        // x = a * 10^(N/2) + b;
        // y = c * 10^(N / 2) + d
        int N = nDigits(x);

        if (N == 1)
            return x * y;

        //Divide
        int digits = (int) Math.pow(10, N / 2);
        long a = x / digits;
        long b = x % digits;

        long c = y / digits;
        long d = y % digits;

        // Solve
        long p1 = multiply(a, c);
        long p2 = multiply(b, d);
        long aux = multiply(a + b, c + d);
        long p3 = aux - p1 - p2;

        return (p1 * (long) Math.pow(10, N) + p3 * (long) Math.pow(10, N / 2) + p2);
    }

    public static void main(String[] args) {
        long x = 1234;
        long y = 5678;
        System.out.println(multiply(x, y));
    }
}