Adding height of binary tree to insert method

Question

I am creating a program that inserts a character (number/letter) into a binary tree. So far, I'm able to produce an output but it's not what I expected. These are the problems I'm encountering:

1. The insert method is not able to print the correct height of the tree. I am not sure where I should insert my height++; statement to get the correct output.

2. The insert method is only able to add nodes to the right.

Expected Output: ht=3 [K=3 L=[K=1 R=[K=2]] R=[K=5 L=[K=4]]]

My Output: ht=4 [K=3 R=[K=1 R=[K=2 R=[K=5 R=[K=4]]]]

(all nodes are only added to the right 'R')

Here are my classes for reference:

Main Class

BST<Character> bst = new BST<>();

bst.insert('3');
bst.insert('1');
bst.insert('2');
bst.insert('5');
bst.insert('4');

System.out.println("ht=" + bst.height + " " + bst.toString());

BST Class - where the insert method is declared

public class BST<T> extends BT<T> {
    // insert() method
    public void insert(char k) 
    {
        if (root == null) {
            root = new BTNode(k);
            return;
        }
        
        BTNode<T> n = root;
        BTNode<T> p = null; // parent
    
        while (n != null) {
            p = n;
            
            if (k < n.value) {
                n = n.left;
            } else {
                n = n.right;
            }
        }
        
        if (k < p.value) {
            p.left = new BTNode(k);
        } else {
            p.right = new BTNode(k);
            height++;   // adds 1 to height when a new level is made
        }
    }
}

BTNode Class

public class BTNode<T> {
    T info;
    int value, level;
    BTNode<T> left, right;
    
    public BTNode(T el) {
        this(el, null, null);
    }
    
    public BTNode(T el, BTNode<T> l, BTNode<T> r) {
        info = el;
        left = l;
        right = r;
    }
}

BT Class - where the toString method is declared

public class BT<T> {
    BTNode<T> root = null;
    int height = 0;
    
    public BT() {
        BTNode<T> node = new BTNode("");
    }
    
    // other methods
    
    // toString()
    public String toString() {
        return toString(root);
    }
    
    public String toString(BTNode<T> n) {
        String s = "";
        
        if (n == null) {
            return "";
        }
        
        if (n != null) {
            s = "[K=" + n.info;
            
            if (n.left != null) {
                s = s + " L=" + toString(n.left) + "]";
            }
            if (n.right != null) {
                s = s + " R=" + toString(n.right) + "]";
            }
        }
        return s;
    }
}

Hope you can help me out, thanks!

Answer 1

You have quite a few issues in your code. I'll list a few immediate items but you really will need to learn to use an interactive debugger and unit testing to resolve the sorts of issues you are seeing.

1. You refer to the value field in BTNode in your comparison but it is never set. You should really be referring to info (which is the actual data in the node).

2. But given info is a generic type you can't use standard comparison operators. Instead you'll need to define it as <T extends Comparable<T>> and then use n.info.compareTo(k) > 0.

3. The key passed into insert should also be of type T

4. Which means the other classes also need to ensure T extends Comparable.

5. Height is only incremented when nodes are added to the right which makes no sense.

Height needs to be increased only when a node is inserted further from the root than the current maximum. Something like the following:

int depth = 0;
while (n != null) {
    depth++;
    p = n;
...
}

depth++;
if (depth > height)
    height = depth;

You should get used to making your fields private and accessing them through getters. In your case a compareValue method would likely make sense.