D20 Answer

/* Puzzle D20 -- fill a 2D array with random ints, without any duplicates
|
|  Return 1 if successful, 0 if not. Failure is guaranteed if there
|  are fewer integers in the range low..high than there are elements
|  in the array.
|
*/
int randomFill2DUnique( int x[][NUMCOLS], int nrows, int low, int high )
{
  int r, c, ri, ci;
  int unique ;
  int trial;
  
  /* Check that the request is possible */
  if ( NUMCOLS*nrows > high-low+1 )
    return 0;
  
  /* Fill the array in raster order */
  for ( r=0; r<nrows; r++ )
    for ( c=0; c<NUMCOLS; c++ )
    {
      /* Find a unique value for the current slot */
      unique = 0;
      while ( !unique )
      {
        /* Pick a trial value for the current slot */
        trial = randInt( low, high );
        unique = 1;
        
        /* Look for the trial in the competed rows */
        for ( ri=0; ri<r && unique; ri++ )
          for ( ci=0; ci<NUMCOLS && unique; ci++ )
            if ( x[ri][ci] == trial ) unique = 0 ;

        /* Look for trial in the current row */
        for ( ci=0; ci<c && unique; ci++ )
          if ( x[r][ci] == trial ) unique = 0 ;
      }
      
      /* Fill the slot with a unique value */
      x[r][c] = trial ;
    }
    
  return 1;
}