Create a function `sierpinski` to generate an ASCII representation of a Sierpinski triangle of order **N**. 
Seperate each line with `\n`. You don't have to check the input value.
The output should look like this: 
     sierpinski(4)
                   *               
                  * *              
                 *   *             
                * * * *            
               *       *           
              * *     * *          
             *   *   *   *         
            * * * * * * * *        
           *               *       
          * *             * *      
         *   *           *   *     
        * * * *         * * * *    
       *       *       *       *   
      * *     * *     * *     * *  
     *   *   *   *   *   *   *   * 
    * * * * * * * * * * * * * * * *
sierpinski=lambda n,pr=['*'],sp=3:sierpinski(n-1,pr+[(j+(' '*len(pr[-1-i]))+j)for i,j in enumerate(pr)],sp*2+1)if n else'\n'.join([i.center(sp//2)for i in pr])
sierpinski=lambda n,pr=['*'],sp=3:sierpinski(n-1,pr+[(j+(' '*len(pr[-1-i]))+j)for i,j in enumerate(pr)],sp*2+1)if n else'\n'.join([i.center(sp//2)for i in pr])