Hi everyone I am having trouble finding the nth node of a binary search tree in an inorder traversal. I am getting NullPointerExceptions for any numbers other than 2 and 3 and even those print out the wrong values. If anyone could help it would be greatly appreciated. The method in question is the nthElement(BinaryNode<AnyType> t, int n). Sorry for the long post. We must use recursion and we are not allowed to use any sort of lists. Thank you very much
package proj2;
//
// CONSTRUCTION: with no initializer
//
// ******************PUBLIC OPERATIONS*********************
// void insert( x ) --> Insert x
// void remove( x ) --> Remove x
// boolean contains( x ) --> Return true if x is present
// Comparable findMin( ) --> Return smallest item
// Comparable findMax( ) --> Return largest item
// boolean isEmpty( ) --> Return true if empty; else false
// void makeEmpty( ) --> Remove all items
// void printTree( ) --> Print tree in sorted order
// ******************ERRORS********************************
// Throws UnderflowException as appropriate
/**
* Implements an unbalanced binary search tree.
* Note that all "matching" is based on the compareTo method.
* @author Mark Allen Weiss
*/
public class BinarySearchTree<AnyType extends Comparable<? super AnyType>>
{
/**
* Construct the tree.
*/
public BinarySearchTree( )
{
root = null;
}
/**
* Insert into the tree; duplicates are ignored.
* @param x the item to insert.
*/
public void insert( AnyType x )
{
root = insert( x, root );
}
public int treeSize(){
return treeSize(root);
}
/**
* Remove from the tree. Nothing is done if x is not found.
* @param x the item to remove.
*/
public void remove( AnyType x )
{
root = remove( x, root );
}
/**
* Find the smallest item in the tree.
* @return smallest item or null if empty.
*/
public AnyType findMin( )
{
if( isEmpty( ) )
throw new UnderflowException( );
return findMin( root ).element;
}
/**
* Find the largest item in the tree.
* @return the largest item of null if empty.
*/
public AnyType findMax( )
{
if( isEmpty( ) )
throw new UnderflowException( );
return findMax( root ).element;
}
public BinaryNode<AnyType> nthElement(int n){
return nthElement(n, root);
}
/**
* Find an item in the tree.
* @param x the item to search for.
* @return true if not found.
*/
public boolean contains( AnyType x )
{
return contains( x, root );
}
/**
* Make the tree logically empty.
*/
public void makeEmpty( )
{
root = null;
}
/**
* Test if the tree is logically empty.
* @return true if empty, false otherwise.
*/
public boolean isEmpty( )
{
return root == null;
}
/**
* Print the tree contents in sorted order.
*/
public void printTree( )
{
if( isEmpty( ) )
System.out.println( "Empty tree" );
else
printTree( root );
}
/**
* Internal method to insert into a subtree.
* @param x the item to insert.
* @param t the node that roots the subtree.
* @return the new root of the subtree.
*/
private BinaryNode<AnyType> insert( AnyType x, BinaryNode<AnyType> t )
{
if( t == null )
return new BinaryNode<AnyType>( x, null, null );
int compareResult = x.compareTo( t.element );
if( compareResult < 0 )
t.left = insert( x, t.left );
else if( compareResult > 0 )
t.right = insert( x, t.right );
else
; // Duplicate; do nothing
return t;
}
/**
* Internal method to remove from a subtree.
* @param x the item to remove.
* @param t the node that roots the subtree.
* @return the new root of the subtree.
*/
private BinaryNode<AnyType> remove( AnyType x, BinaryNode<AnyType> t )
{
if( t == null )
return t; // Item not found; do nothing
int compareResult = x.compareTo( t.element );
if( compareResult < 0 )
t.left = remove( x, t.left );
else if( compareResult > 0 )
t.right = remove( x, t.right );
else if( t.left != null && t.right != null ) // Two children
{
t.element = findMin( t.right ).element;
t.right = remove( t.element, t.right );
}
else
t = ( t.left != null ) ? t.left : t.right;
return t;
}
/**
* Internal method to find the smallest item in a subtree.
* @param t the node that roots the subtree.
* @return node containing the smallest item.
*/
private BinaryNode<AnyType> findMin( BinaryNode<AnyType> t )
{
if( t == null )
return null;
else if( t.left == null )
return t;
return findMin( t.left );
}
/**
* Internal method to find the largest item in a subtree.
* @param t the node that roots the subtree.
* @return node containing the largest item.
*/
private BinaryNode<AnyType> findMax( BinaryNode<AnyType> t )
{
if( t != null )
while( t.right != null )
t = t.right;
return t;
}
/**
* Internal method to find an item in a subtree.
* @param x is item to search for.
* @param t the node that roots the subtree.
* @return node containing the matched item.
*/
private boolean contains( AnyType x, BinaryNode<AnyType> t )
{
if( t == null )
return false;
int compareResult = x.compareTo( t.element );
if( compareResult < 0 )
return contains( x, t.left );
else if( compareResult > 0 )
return contains( x, t.right );
else
return true; // Match
}
/**
* Internal method to print a subtree in sorted order.
* @param t the node that roots the subtree.
*/
private void printTree( BinaryNode<AnyType> t )
{
if( t != null )
{
printTree( t.left );
System.out.println( t.element );
printTree( t.right );
}
}
/**
* Internal method for finding tree size.
* @param t the node that roots the subtree.
* @return the number of nodes.
*/
private int treeSize(BinaryNode<AnyType> t){
int size = 1;
if(t.right != null){
size = size + treeSize(t.right);
}
if(t.left != null){
size = size + treeSize(t.left);
}
return t.treeSize = size;
}
/**
* Internal method for traversing the tree in-order.
* @param t the node that roots the subtree.
* @return
*/
private BinaryNode<AnyType> nthElement(int n, BinaryNode<AnyType> t){
if(t.left.treeSize == n){
return t;
}else if(t.left.treeSize > n){
return nthElement(n, t.left);
}else if(t.left.treeSize < n){
int k = t.treeSize - t.left.treeSize;
return nthElement(k, t.right);
}
return t;
}
/**
* Internal method to compute height of a subtree.
* @param t the node that roots the subtree.
*/
private int height( BinaryNode<AnyType> t )
{
if( t == null )
return -1;
else
return 1 + Math.max( height( t.left ), height( t.right ) );
}
// Basic node stored in unbalanced binary search trees
private static class BinaryNode<AnyType>
{
// Constructors
BinaryNode( AnyType theElement )
{
this( theElement, null, null );
}
BinaryNode( AnyType theElement, BinaryNode<AnyType> lt, BinaryNode<AnyType> rt )
{
element = theElement;
left = lt;
right = rt;
}
int treeSize;
AnyType element; // The data in the node
BinaryNode<AnyType> left; // Left child
BinaryNode<AnyType> right; // Right child
}
/** The tree root. */
private BinaryNode<AnyType> root;
// Test program
public static void main( String [ ] args )
{
BinarySearchTree<Integer> t = new BinarySearchTree<Integer>( );
t.insert(55);
t.insert(40);
t.insert(35);
t.insert(37);
t.insert(60);
t.insert(70);
t.insert(80);
t.insert(69);
System.out.println("this is tree size: " + t.treeSize());
t.printTree();
System.out.println(t.nthElement(3).element);
}
}