Implementing a Binary Tree

Question

I'm trying to implement a binary tree, but its just not taking the root. Any ideas? It looks like the root should be inserting fine, but I'm just getting a null when I print it. Am I trying to add only temporary nodes to the tree that don't "stick"?

public class tree {
    public static void main(String args[]){
        Treeb tree = new Treeb();
        tree.add(10);
        tree.add(20);
        tree.add(2);
        tree.add(6);
        tree.printTree();
    }
}
class Node{
    int data;
    Node left;
    Node right;
    public Node(int data){
        this.data = data;
        left = null;
        right = null;
    }
    Node getLeft(){
        return left;
    }
    Node getRight(){
        return right;
    }
}

class Treeb{
    Node root;
    Treeb(){
        root = null;
    }

    void add(int n){
        addNode(n, root);
    }

    void addNode(int n, Node vert){
        if(vert == null){
            vert = new Node(n);
        }
        else if(vert.left.data < n){
            if(vert.left == null){
                vert.left = new Node(n);
            }
            else{
                addNode(n, vert.left);
            }
        }
        else if(vert.right.data >= n){
            if(vert.right == null){
                vert.right = new Node(n);
            }
            else{
                addNode(n,vert.right);
            }
        }
    }
    void printTree(){
        if(root != null){
            printChild(root);
        }
        System.out.println(root);
    }
    void printChild(Node leaf){
        System.out.print(leaf.data);
        if(leaf.left != null){
            printChild(leaf.getLeft());
        }
        if(leaf.right != null){
            printChild(leaf.getRight());
        }
    }
}

Hi, two (small) remarks beside the answers: class names begin with capital letter and it should be nice if you sometimes used modifiers such as private. Furthermore, when you create getters ( getLeft() and getRight()) it is always a good idea to use them instead of instance variables. (This, of course, was made possible because you didn't use 'private' modifier.) — C.Champagne
– C.Champagne, Commented May 7, 2013 at 7:42

Alexander Rühl · Accepted Answer · 2013-05-07 07:31:32Z

4

You are assigning vert a new reference, but not root, that's why it stays null.

answered May 7, 2013 at 7:31

Alexander Rühl

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Comments

Étienne Miret · Accepted Answer · 2013-05-07 07:35:27Z

3

Your method addNode(int, Node) isn't working.

First:

if(vert == null){
    vert = new Node(n);
}

You're assigning the new node to a local variable. Thus, the new Node gets discarded at the end of the method.

Second:

}
else if(vert.left.data < n){
    // code
}
else if(vert.right.data >= n){

vert.left and vert.rightcan be null, so you would get an NPE when vert is not null.

answered May 7, 2013 at 7:35

Étienne Miret

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Comments

Maroun · Accepted Answer · 2013-05-07 07:37:16Z

3

getLeft() and getRight() can (and will) return null sometime. You should make sure in your printChild() that leaf itself is not null. (You're probably getting NPE in if(leaf.left != null) since leaf is null), you might also want to reconsider your tree construction again, root is null in your case.

edited May 7, 2013 at 7:37

answered May 7, 2013 at 7:29

Maroun

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