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import java.util.*;
import java.io.File;
import java.util.regex.Pattern;

// I don't know how your FileDescription class is defined
// but I just make a simplified example here
class FileDescription
{
    private File file;
    public FileDescription(String s) { file = new File(s); }
    public File getFile() { return file; }
}

// DirTree = anything in the filesystem, including file or directory
class DirTree
{
    // if it is a non-empty directory, contents will be non-null
    public Map<String,DirTree> contents;
    // if it is a file given in the input, then description will be non-null
    public FileDescription description;

    public void print() { print("", 0); }
    // recursively print directory tree
    public void print(String prefix, int depth) {
        for (int i = 0; i < depth; i++)
            System.out.print("-");
        System.out.println(prefix);

        if (contents != null)
            for (Map.Entry<String,DirTree> e : contents.entrySet()) {
                String name = e.getKey();
                DirTree subTree = e.getValue();

                subTree.print(prefix + "\\" + name, depth + 1);
            }
    }
}

public class PreviewTreeNode
{
    public static DirTree generateTree(Collection<FileDescription> files) {
        DirTree root = new DirTree();

        for (FileDescription fd : files) {
            String path = fd.getFile().getAbsolutePath();

            // keeps track of location in tree
            DirTree current = root;
            // iterate through each directory in path
            for (String part : path.split(Pattern.quote("\\"))) {
                if (current.contents == null)
                    current.contents = new HashMap<String,DirTree>();
                // add it if it is not already in the directory tree
                if (!current.contents.containsKey(part))
                    current.contents.put(part, new DirTree());
                // go in to the next deeper level
                current = current.contents.get(part);
            }

            // now current refers to the location of the file we want
            current.description = fd;
        }

        return root;
    }


    public static void main(String[] args) {
        Collection<FileDescription> files =
            Arrays.asList(new FileDescription("A:\\Folder1\\Folder2\\Folder3\\File1.txt"),
                          new FileDescription("A:\\Folder1\\File2.txt"),
                          new FileDescription("B:\\Folder4\\Folder5\\File3.txt"),
                          new FileDescription("B:\\Folder4\\Folder5\\File4.txt"));

        DirTree root = generateTree(files);

        root.print();
    }
}