Copying a Binary Tree in Java using a preorder traversal

Question

I am trying to copy a binary tree using the pre order traversal but I am stuck. As I am not putting any of the values into a new tree they are obviously not copying correctly...

public class Node{

int key;
String name;

Node leftChild;
Node rightChild;

Node(int key, String name){
    this.key = key;
    this.name = name;

}



public class BinaryTree{

public Node root;

public void copyTree(Node focusNode){

    if(focusNode != null){

        Node copyNode = new Node(focusNode.key, focusNode.name);

        //System.out.println(copyNode);

        copyTree(focusNode.leftChild);
        copyTree(focusNode.rightChild);
    }
}

}

Answer 1

Here's one solution. I added a toString() method to the Node class for presentation purposes.

class Node {

    int key;
    String name;

    Node leftChild;
    Node rightChild;

    Node(int key, String name) {
        this.key = key;
        this.name = name;
    }

    public String toString() {
        return "[" + key + "," + name + "]";
    }
}

The BinaryTree was slightly modified as well:

class BinaryTree {

    public Node root;

    public BinaryTree copyTree(Node focusNode) {
        BinaryTree bt = new BinaryTree();
        bt.root = preOrderCopy(focusNode);
        return bt;
    }

    public static void preOrderPrint(BinaryTree t) {
        preOrderPrint(t.root);
    }

    public static void preOrderPrint(Node n) {
        if (n == null) {
            // base case
            return;
        }
        System.out.println(n);
        preOrderPrint(n.leftChild);
        preOrderPrint(n.rightChild);

    }

    private Node preOrderCopy(Node focusNode) {
        if (focusNode == null) {
            // base case
            return null;
        }
        Node copy = new Node(focusNode.key, focusNode.name);
        copy.leftChild = preOrderCopy(focusNode.leftChild);
        copy.rightChild = preOrderCopy(focusNode.rightChild);
        return copy;
    }

}

To test the code, I created a BinaryTree based on the one shown on the Wikipedia page for Tree Traversal. Here's a picture of the tree used in this example:

The proper Pre-order traversal for this example is : F, B, A, D, C, E, G, I, H. You can use the following code to test this implementation:

public class NodeTest {

    public static void main(String[] args) {
        BinaryTree bt = new BinaryTree();
        Node a = new Node(1, "A");
        Node b = new Node(2, "B");
        Node c = new Node(3, "C");
        Node d = new Node(4, "D");
        Node e = new Node(5, "E");
        Node f = new Node(6, "F");
        Node g = new Node(7, "G");
        Node h = new Node(8, "H");
        Node i = new Node(9, "I");
        f.leftChild = b;
        b.leftChild = a;
        b.rightChild = d;
        d.leftChild = c;
        d.rightChild = e;
        f.rightChild = g;
        g.rightChild = i;
        i.leftChild = h;
        bt.root = f;

        System.out.println("Print full tree:");
        BinaryTree.preOrderPrint(bt.copyTree(f));

        System.out.println("Only print f's left sub-tree:");
        BinaryTree.preOrderPrint(bt.copyTree(f.leftChild));

    }

}

Running the above code produces the following output:

Print full tree:
[6,F]
[2,B]
[1,A]
[4,D]
[3,C]
[5,E]
[7,G]
[9,I]
[8,H]
Only print f's left sub-tree:
[2,B]
[1,A]
[4,D]
[3,C]
[5,E]