Tree creation algorithm needed. Experts pls help

Question

hi,
I'm writing a program that read data from database to create a Tree.
The database table:
DATA          PREDATA          POSTDATA
1 4 5
2 6 7
3 1 2
4 8 9
If the DATA has a PREDATA, it will be inserted into the left node otherwise null and POSTDATA will be inserted into right node. For example if I pass in 3 the Tree should look like the following:
3
1 2
4 5 6 7
8 9 null null null
Thanks

Answer 1

This is a fairly straighforward problem but I see two possible problems.
1) There could be more than one root. Any row with identifier that is never used as pre or post is a root.
2) If a row identifier is used more than once as pre or post then one no longer has a simple binary tree.
Dealing with 1) is easy - just have a set of roots. Dealing with 2) is not so easy.
My solution follows. It deals with 1 but not 2.
import java.util.*;
public class Test20040622
    static String[][] testData =
        {"1","4","5"},
        {"2","6","7"},
        {"3","1","2"},
        {"4","8","9"},
    static class TreeElement
        TreeElement(String value, Object pre, Object post)
            this.value = value;
            this.pre = pre;
            this.post = post;
        public String toString()
            return value;
        String value;
        Object pre;
        Object post;
    private Set treeRoots;
    public Test20040622()
        HashMap map = new HashMap();
        // Build a map pointing from id to TreeElement
        for (int rowIndex = 0; rowIndex < testData.length; rowIndex++)
            String[] row = testData[rowIndex];
            map.put(row[0], new TreeElement(row[0], row[1], row[2]));
        // The set of keys to the TreeElements
        treeRoots = new HashSet(map.keySet());
        // Go through each of the elements replacing the
        // pre and post String with the corresponding TreeElement
        for (Iterator it = map.values().iterator(); it.hasNext();)
            TreeElement treeElement = (TreeElement)it.next();
            if (treeElement.pre instanceof String)
                treeRoots.remove(treeElement.pre);
                Object pre = map.get(treeElement.pre);
                if (pre != null)
                    treeElement.pre = pre;
            if (treeElement.post instanceof String)
                treeRoots.remove(treeElement.post);
                Object post = map.get(treeElement.post);
                if (post != null)
                    treeElement.post = post;
        // Convert the tree root Strings to TreeElements
        HashSet roots = new HashSet();
        for (Iterator it = treeRoots.iterator(); it.hasNext();)
            roots.add(map.get(it.next()));
        treeRoots = roots;
    public void print()
        for (Iterator it = treeRoots.iterator(); it.hasNext();)
            printOne(it.next(), 0);
    private void printOne(Object leaf, int level)
        if (leaf instanceof TreeElement)
            printOne( ((TreeElement)leaf).pre, level+1);
        for (int i = 0; i < level; i++)
            System.out.print("   ");
        System.out.println(leaf);
        if (leaf instanceof TreeElement)
            printOne(((TreeElement)leaf).post, level+1);
    public static void main(String[] args)
        new Test20040622().print();
}