disjsets.java

Data StructuresAnd Algorithm Analysis In Java Source Code

    package DataStructures;

    // DisjSets class
    //
    // CONSTRUCTION: with int representing initial number of sets
    //
    // ******************PUBLIC OPERATIONS*********************
    // void union( root1, root2 ) --> Merge two sets
    // int find( x )              --> Return set containing x
    // ******************ERRORS********************************
    // No error checking is performed

    /**
     * Disjoint set class. (Package friendly so not used accidentally)
     * Does not use union heuristics or path compression.
     * Elements in the set are numbered starting at 0.
     * @author Mark Allen Weiss
     * @see DisjSetsFast
     */
    class DisjSets
    {
        /**
         * Construct the disjoint sets object.
         * @param numElements the initial number of disjoint sets.
         */
        public DisjSets( int numElements )
        {
            s = new int [ numElements ];
            for( int i = 0; i < s.length; i++ )
                s[ i ] = -1;
        }

        /**
         * Union two disjoint sets.
         * For simplicity, we assume root1 and root2 are distinct
         * and represent set names.
         * @param root1 the root of set 1.
         * @param root2 the root of set 2.
         */
        public void union( int root1, int root2 )
        {
            s[ root2 ] = root1;
        }

        /**
         * Perform a find.
         * Error checks omitted again for simplicity.
         * @param x the element being searched for.
         * @return the set containing x.
         */
        public int find( int x )
        {
            if( s[ x ] < 0 )
                return x;
            else
                return find( s[ x ] );
        }

        private int [ ] s;


        // Test main; all finds on same output line should be identical
        public static void main( String [ ] args )
        {
            int numElements = 128;
            int numInSameSet = 16;

            DisjSets ds = new DisjSets( numElements );
            int set1, set2;

            for( int k = 1; k < numInSameSet; k *= 2 )
            {
                for( int j = 0; j + k < numElements; j += 2 * k )
                {
                    set1 = ds.find( j );
                    set2 = ds.find( j + k );
                    ds.union( set1, set2 );
                }
            }

            for( int i = 0; i < numElements; i++ )
            {
                System.out.print( ds.find( i )+ "*" );
                if( i % numInSameSet == numInSameSet - 1 )
                    System.out.println( );
            }
            System.out.println( );
        }
    }