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;
    }
}