Enter no.of rows if input is 3 then output like this

                                   *
                                 * * *
                               * * * * * 
                             *           *  
                           * *           * *
                         * * *           * * *
                           * *           * *
                             *           * 
                               * * * * *
                                 * * *
                                   *
                        If no.of row is 4 then 
                          output is like this:
                                   *
                                 * * *
                               * * * * * 
                             * * * * * * *
                           *               *
                         * *               * *  
                       * * *               * * *
                     * * * *               * * * *
                       * * *               * * *
                         * *               * * 
                           *               *
                             * * * * * * *
                               * * * * *
                                 * * *
                                   *

what is the logic behind this structure as per input?