Question

This is a homework assignment. I try to find the number of nodes in a tree structure that contains even number data values. The following is my failed attempt in a Java class named LinkedTree.

public int numEvenKeys() {
        return numEvenKeysTree(root);     
}

private static int numEvenKeysTree(Node root) {
        int num = 0;

        if (root == null)
            return 0;        
        else if (root.left != null || root.right != null)
            return num + numEvenKeysTree(root.left) 
                    + numEvenKeysTree(root.right);
        else if (root.key % 2 == 0)
            num = 1;

        return num;
}

Here is part of my main class:

public static void main(String[] args) {
                Scanner in = new Scanner(System.in);

                LinkedTree tree = new LinkedTree();
                tree.insert(7, "root node");
                tree.insert(9, "7's right child");
                tree.insert(5, "7's left child");
                tree.insert(2, "5's left child");
                tree.insert(8, "9's left child");
                tree.insert(6, "5's right child");
                tree.insert(4, "2's right child");
                   ...
                *** remove a node of your choice ***
                   ...
                System.out.print("number of nodes with even keys in this tree: ");
                System.out.println(tree.numEvenKeys());
                   ...
    } 

As a reference, here is the inner class node and class constructor:

private class Node {
    private int key;         // the key field
    private LLList data;     // the data items associated with this key
    private Node left;       // reference to the left child/subtree
    private Node right;      // reference to the right child/subtree
    private Node parent;     // reference to the parent

    private Node(int key, Object data, Node left, Node right, Node parent){
        this.key = key;
        this.data = new LLList();
        this.data.addItem(data, 0);
        this.left = left;
        this.right = right;
        this.parent = parent;
    }

    private Node(int key, Object data) {
        this(key, data, null, null, null);
    }
}

// the root of the tree as a whole
private Node root;

public LinkedTree() {
    root = null;
}

The tree has a structure of:

      7
     / \
    5   9
   / \ / 
  2  6 8
   \
    4

If I choose to remove node 7, the method should return 4. However, it returns 1 with my implementation. Any suggestions?

Answer 1

You got your conditions wrong.

If the node is null, then the answer is 0.

If the node is even, it should be 1 + the number of even nodes in the left subtree + the number of even nodes in the right subtree.

If the node is odd, it should be 0 + the number of even nodes in the left subtree + the number of even nodes in the right subtree.