阶梯算法··该如何处理

public class Test {    public static void main(String[] args) {        System.out.println("1 * 2 * 3 * 4 * 5 = " + factorial(5));        System.out.println("3 * 4 * 5 = " + factorial(3, 5));    }        /**     * 计算 num 的阶乘     * @param num     * @return     */    private static int factorial(int num) {                return factorial(1, num);    }        /**     * 计算 start * (start + 1) * (start + 2) * ... * end 的值     * @param start     阶乘的起始数     * @param end       阶乘的结束数     * @return     */    private static int factorial(int start, int end) {        if(start > end) {            int t = start;            start = end;            end = t;        }        int result = start > 0 ? start : 1;        while(end > start) {            result *= end--;        }        return result;    }}