Question

EDIT: proper solution:

void add(Student s)
{
    if(root == null)
        root = new TreeNode(s);
    else
        insert(root,s);        
}

void insert(TreeNode tn, Student s)
{
    if(tn.sVal.compareTo(s) > 0)
    {
        if(tn.left != null)
            insert(tn.left, s);            
        else
        {
            tn.left = new TreeNode(s);
            tn.left.parent = tn;
        }
    }
    else if(tn.sVal.compareTo(s) < 0)
    {
        if(tn.right != null)
            insert(tn.right, s);
        else
        {
            tn.right = new TreeNode(s);
            tn.right.parent = tn;
        }
    }
    balance(tn);
}

---

I’m trying to inserting to a binary tree using the following:

void add(Student s)
    {
        insert(root,s);
    }

private void insert(TreeNode t, Student s)
{        
    if(t == null)
        t = new TreeNode(s);        
    else if(t.sVal.compareTo(s) > 0)
        insert(t.right, s);
    else if(t.sVal.compareTo(s) < 0)
        insert(t.left,s);                
}

However, the tree remains empty, and I can’t figure out why. I hate to be so vague, but I can’t find an error in the logic. What am I missing?

Answer 1

Here’s a big hint: Make this change first, then debug from there:

if (t == null)
    throw new IllegalArgumentException();

And a still bigger hint: When you create the new node, you must also have a reference to its parent so you can add it to the parent.