code: public static long factorial(long start, long end) {
  		if (start < end) {
  			return 0L;
  		}
  		if (start == end) {
  			return 1L;
  		}
  		return start * factorial(start - 1, end);
  	}
docs: 计算阶乘
  <p>
  n! = n * (n-1) * ... * end
  </p>
  
  @param start 阶乘起始
  @param end 阶乘结束
  @return 结果
  @since 4.1.0