// sanityCheck()
   //
   // This method makes sure that the HeapItems in H[] are heap ordered.
   // It also checks some other heap properties.
   // It does not check all heap properties (e.g., that items are filled
   // level by level).
   //
   public void sanityCheck() {
      boolean bad = false ;
      int left, right ;

      for (int i = 1 ; i <= maxSize ; i++) {

         left = 2 * i ; 
         right = 2 * i + 1 ;

         if ( H[i].tag == EMPTY ) {

            // an empty node should not have non-empty children

            if ( left <= maxSize && H[left].tag != EMPTY ) {
               bad = true ;
               System.out.println("*** Non-empty left child of empty node at index: " + i) ;
            }

            if ( right <= maxSize && H[right].tag != EMPTY ) {
               bad = true ;
               System.out.println("*** Non-empty right child of empty node at index: " + i) ;
            }
         } else {

             // check that left child is smaller than parent

            if ( left <= maxSize && (H[left].tag != EMPTY) &&  (H[left].priority > H[i].priority) ) {
               bad = true ;
               System.out.println("*** Left child bigger at index: " + i) ;
            }

            // check that right child is smaller than parent

            if ( right <= maxSize && (H[right].tag != EMPTY) &&  (H[right].priority > H[i].priority) ) {
               bad = true ;
               System.out.println("*** Right child bigger at index: " + i) ;
            }

            // if there's no left child, there shouldn't be a right child

            if ( left <= maxSize &&  right <= maxSize && H[left].tag == EMPTY && H[right].tag != EMPTY ) {
               bad = true ;
               System.out.println("*** Left child empty but right child non-empty at index: " + i) ;
            }
         }

      }  // end of for loop


      if (!bad) {
         System.out.println("Heap passes sanity check!!!") ;
      }
   }