public static void compress() {
        // read the input
        String s = BinaryStdIn.readString();
        char[] input = s.toCharArray();

        // tabulate frequency counts
        int[] freq = new int[R];
        for (int i = 0; i < input.length; i++)
            freq[input[i]]++;

        // build Huffman trie
        Node root = buildTrie(freq);

        // build code table
        String[] st = new String[R];
        buildCode(st, root, "");

        // print trie for decoder
        writeTrie(root);

        // print number of bytes in original uncompressed message
        BinaryStdOut.write(input.length);

        // use Huffman code to encode input
        for (int i = 0; i < input.length; i++) {
            String code = st[input[i]];
            for (int j = 0; j < code.length(); j++) {
                if (code.charAt(j) == '0') {
                    BinaryStdOut.write(false);
                }
                else if (code.charAt(j) == '1') {
                    BinaryStdOut.write(true);
                }
                else throw new IllegalStateException("Illegal state");
            }
        }

        // close output stream
        BinaryStdOut.close();
    }