因式分解

	/**
	 * 因式分解，如果为素数则返回“”
	 * 
	 * @param num
	 * @return
	 */
	public static String factorize(int num) {

		int m = num = num >= 2 ? num : 2;
		String factors = "";

		for (int i = 2; (m > 1) && (i * i <= num);) {

			if (m % i != 0) {
				i++;
				continue;
			}
			m /= i;
			factors = (factors.equals("")) ? "" + i : factors + " * " + i;
		}

		if (1 < m && m < num) {
			factors = factors + " * " + m;
		}

		return factors;
	}