Heapsort

Heapsort is one of the best general-purpose sorting algorithms, a comparison sort and part of the selection sort family. Although somewhat slower in practice on most machines than a good implementation of quicksort, it has the advantages of worst-case O(n log n) runtime and being an in-place algorithm. Heapsort is not a stable sort.

 

/**
     * Standard heapsort.
     * @param a an array of Comparable items.
     */
    public static void heapsort( Comparable [ ] a )
    {
        for( int i = a.length / 2; i >= 0; i-- )  /* buildHeap */
            percDown( a, i, a.length );
        for( int i = a.length - 1; i > 0; i-- )
        {
            swapReferences( a, 0, i );            /* deleteMax */
            percDown( a, 0, i );
        }
    }

    /**
     * Internal method for heapsort.
     * @param i the index of an item in the heap.
     * @return the index of the left child.
     */
    private static int leftChild( int i )
    {
        return 2 * i + 1;
    }

    /**
     * Internal method for heapsort that is used in
     * deleteMax and buildHeap.
     * @param a an array of Comparable items.
     * @index i the position from which to percolate down.
     * @int n the logical size of the binary heap.
     */
    private static void percDown( Comparable [ ] a, int i, int n )
    {
        int child;
        Comparable tmp;

        for( tmp = a[ i ]; leftChild( i ) < n; i = child )
        {
            child = leftChild( i );
            if( child != n - 1 && a[ child ].compareTo( a[ child + 1 ] ) < 0 )
                child++;
            if( tmp.compareTo( a[ child ] ) < 0 )
                a[ i ] = a[ child ];
            else
                break;
        }
        a[ i ] = tmp;
    }
    
    
    /**
     * Method to swap to elements in an array.
     * @param a an array of objects.
     * @param index1 the index of the first object.
     * @param index2 the index of the second object.
     */
    public static final void swapReferences( Object [ ] a, int index1, int index2 )
    {
        Object tmp = a[ index1 ];
        a[ index1 ] = a[ index2 ];
        a[ index2 ] = tmp;
    }