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Deleting a node at a given index in Java; What am I doing wrong?

I have been stuck for the last 40-so minutes on this problem. I'm unsure of what I am doing wrong here, I have tried debugging many times and have read similar Q/A's. Can't figure it out and my assignment is due at midnight.
It is not deleting the Node.. None at all in fact. If the code seems right, then the problem may be lying inside main. Let me know, please & thank you!
Any help is well appreciated! :)
Problem Method Below
void deleteAtIndex(int idx)
{
if (length() >= idx)
{
if(idx == 0)
{
head = head.getNext();
}
else
{
Node temp = findAtIndex(idx-1);
temp.setNext(temp.getNext().getNext());
}
}
else
System.out.println("Invalid position");
}
Full Class Code Below
public class ShapeLinkedList {
public Node head; // Head is first node in linked list
public ShapeLinkedList() { }
public ShapeLinkedList(Node head) {
}
public boolean isEmpty() {
return length() == 0;
}
public void insertAtEnd(Shape data) {
Node end = new Node(data, null);
if (head == null)
head.setNext(end);
else
tail().setNext(end);
}
public void insertAtBeginning(Shape data) {
if (data != null)
{
Node temp = new Node(data, head);
head = temp;
}
}
public Node tail() {
if (head == null){
return null;
}
else
{
Node temp = head;
while(temp.getNext() != null){
temp = temp.getNext();
}
return temp;
}
}
public int length() { // 1
Node temp = head;
if (temp == null)
return 0;
int tempIndex = 0;
while(temp.getNext() != null){
temp = temp.getNext();
tempIndex++;
}
return tempIndex;
}
void insertAtIndex(int idx, Shape data) { //3
if (length() >= idx)
{
Node current = new Node(data, null);
Node temp = findAtIndex(idx);
current.setNext(temp.getNext());
temp.setNext(current);
}
}
Node findAtIndex(int idx) { // 2
if (length() >= idx)
{
Node temp = head;
for(int i = 0; i < idx; i++)
{
temp = temp.getNext();
}
return temp;
}
else
return null;
}
void deleteAtIndex(int idx) { //4
if (length() >= idx)
{
if(idx == 0)
{
head = head.getNext();
}
else
{
Node temp = findAtIndex(idx-1);
temp.setNext(temp.getNext().getNext());
}
}
else
System.out.println("Invalid position");
}
#Override
public String toString() {
return "";
}
void deleteData(Shape s) {
Node temp = head;
for(int i = 0; i < length(); i++)
{
if(temp.getData() == s)
deleteAtIndex(i);
}
}
#Override
public int hashCode() {
return 0;
}
#Override
public boolean equals(Object obj) {
return false;
}
// Node is nested class because it only exists along with linked list
public static class Node {
private Shape data;
private Node next;
public Node(Shape S, Node N){
data = S;
next = N;
}
public Node getNext(){
return next;
}
public Shape getData() { return data; }
public void setNext(Node N) { next = N; }
// TODO develop all the methods that are needed
// such as constructors, setters, getters
// toString, equals, hashCode
}
}

You have multiple bugs in your class. I guess if you fix them, your code will work fine. SO is not a debugging platform, thus your question is off-topic and you should learn using a debugger.
Here some issues I instantly have seen from a quick look at your code, I'm sure there are more issues:
if (length() >= idx) should probably be if (length() > idx) at all places and
if (head == null) head.setNext(end); should probably be if (head == null) head = end; and
public ShapeLinkedList(Node head) { } should probably be public ShapeLinkedList(Node head) { this.head = head; } and
int tempIndex = 0 should probably be int tempIndex = 1

Implementing Singly Linked List

I am trying to learn data structure, but I ran into the dreaded NullPointerException and I am not sure how to fix it.
My SinglyLinkedList<E> class implements an interface, LinkedList, where I redefined some methods like, add(), get(), contains(), and more.
The NullPointerException happens when I use the clear() method. It points at the method removeLast() under nodeBefore.setNext(null). It also points to the clear() method under remove(head.getElement()).
Also, if there is anything I can improve upon in my code please let me know.
public class SinglyLinkedList<E> implements LinkedList<E> {
private class Node<E> {
public Node<E> next;
public E element;
public Node(E element) {
this.element = element;
}
public Node (E element, Node<E> next) {
this.element = element;
this.next = next;
}
public E getElement() {
return element;
}
public Node<E> getNext() {
return next;
}
public void setElement(E element) {
this.element = element;
}
public void setNext(Node<E> next) {
this.next = next;
}
public String toString() {
return ("[" + element + "] ");
}
}
public Node<E> head;
public Node<E> tail;
public int total;
public SinglyLinkedList() {
this.head = null;
this.tail = null;
this.total = 0;
}
public E get(int index) {
if (index < 0 || index > size()) {
return null;
}
if (index == 0) {
return head.getElement();
}
Node<E> singly = head.getNext();
for (int i = 1; i < index; i ++) {
if (singly.getNext() == null) {
return null;
}
singly = singly.getNext();
}
System.out.println("\n" + singly.getElement());
return singly.getElement();
}
public void add(E element) {
Node<E> singlyAdd = new Node<E>(element);
if (tail == null) {
head = singlyAdd;
tail = singlyAdd;
} else {
tail.setNext(singlyAdd);
tail = singlyAdd;
}
total++;
}
public void display() {
if (head == null) {
System.out.println("empty list");
} else {
Node<E> current = head;
while (current != null) {
System.out.print(current.toString());
current = current.getNext();
}
}
}
public boolean contains(E data) {
if (head == null) {
return false;
}
if (head.getElement() == data) {
System.out.println(head);
return true;
}
while (head.getNext() != null) {
head = head.getNext();
if (head.getElement() == data) {
System.out.println(head);
return true;
}
}
return false;
}
private Node<E> removeFirst() {
if (head == null) {
System.out.println("We cant delete an empty list");
}
Node<E> singly = head;
head = head.getNext();
singly.setNext(null);
total--;
return singly;
}
private Node<E> removeLast() {
Node<E> nodeBefore;
Node<E> nodeToRemove;
if (size() == 0) {
System.out.println("Empty list");
}
nodeBefore = head;
for (int i = 0; i < size() - 2; i++) {
nodeBefore = nodeBefore.getNext();
}
nodeToRemove = tail;
nodeBefore.setNext(null);
tail = nodeBefore;
total--;
return nodeToRemove;
}
public E remove(int index) {
E hold = get(index);
if (index < 0 || index >= size()) {
return null;
} else if (index == 0) {
removeFirst();
return hold;
} else {
Node<E> current = head;
for (int i = 1; i < index; i++) {
current = current.getNext();
}
current.setNext(current.getNext().getNext());
total--;
return hold;
}
}
public int size() {
return getTotal();
}
public boolean remove(E data) {
Node<E> nodeBefore, currentNode;
if (size() == 0) {
System.out.println("Empty list");
}
currentNode = head;
if (currentNode.getElement() == data) {
removeFirst();
}
currentNode = tail;
if (currentNode.getElement() == data) {
removeLast();
}
if (size() - 2 > 0) {
nodeBefore = head;
currentNode = head.getNext();
for (int i = 0; i < size() - 2; i++) {
if (currentNode.getElement() == data) {
nodeBefore.setNext(currentNode.getNext());
total--;
break;
}
nodeBefore = currentNode;
currentNode = currentNode.getNext();
}
}
return true;
}
public void clear() {
while (head.getNext() != null) {
remove(head.getElement());
}
remove(head.getElement());
}
private int getTotal() {
return total;
}
}

For your clear method, I don't see that you do any per element cleanup, and the return type is void, so all you want is an empty list. The easiest way is to simply clear everything, like in the constructor:
public void clear() {
this.head = null;
this.tail = null;
this.total = 0;
}
Another comment:
in contains, you do
while (head.getNext() != null) {
head = head.getNext();
if (head.getElement() == data) {
System.out.println(head);
return true;
}
}
which may have two problems (where the first applies to the entire class),
you compare with == data which compares references, where you probably want to compare values with .equals(data)
Edit: I.e. n.getElement().equals(data) instead of n.getElement() == data.
(Or, if n and data may be null, something like (data != null ? data.equals(n.getElement()) : data == n.getElement())
you use the attribute head as the scan variable which modifies the state of the list. Do you really want that?

The problem arises when you delete the last element within clear: remove(head.getElement());. For some reason, you first remove the head and then the tail. But when calling removeLast, you use the head (which is already null). Within removeLast this is the line, which causes the NullPointerException: nodeBefore.setNext(null);.
My advice would be to write the clear() method as #bali182 has suggested:
public void clear() {
this.head = null;
this.tail = head;
this.total = 0;
}
One advice: if you are writing methods to search or delete entries, you should never use == when dealing with objects (or even better: don't use == at all when dealing with objects). You may want to read this thread.

From within clear method, you are calling remove(head.getElement()); meaning you are trying to call LinkedList's remove method. And since you are overriding each functionality and so is add, you don't maintain internal state of LinkedList and hence you get exception. Code in remove is:
public boolean remove(Object o) {
if (o==null) {
for (Entry<E> e = header.next; e != header; e = e.next) {
if (e.element==null) {
So here since you are not using functionality of LinkedList, header would be null and doing header.next would return NullPointerException.

Circular Linked List In Ascending Order Java

My task is to implement a circular linked list in java (ascending order) but the problem is that it is going in an infinite loop
I have created a class of Node in which i have define two elements.
public class Node {
public int element;
public Node next;
public class Node {
int element;
Node next;
}
}
Now in the second class of List i have made a insert function i have define a Node head=null in the start and create a new nNode .After that i am checking in the head section if head==null then the first element will be nNode. After inserting the first element i will compare the next element and the head element if the head element is greater than it will shift next and the new nNode will be the head. Since it is the circular linked list it is working but it is also going in an infinite loop.
This is the List class in which i have use the node class variables
public class List {
void insert(int e) {
Node nNode = new Node();
Node tNode = head;
nNode.element = e;
if (head == null)
head = nNode;
else if (head.element > e) {
nNode.next = head;
head=nNode;
} else {
Node pNode = head;
while (tNode.next != head && tNode.element <= e) {
pNode = tNode;
tNode = tNode.next;
}
pNode.next = nNode;
nNode.next = tNode;
tNode.next=head;
}
}
}

I have created on sample program for circular linkedlist which hold name and age of given element.
It has add(), remove() and sorbasedOnAge() (Sorting is implemented by first getting clone and convert it into simple linked list. Then use merge sort so that performance of O(nLogn) could be achieved.)
If you like it don't forget to press like button.
package com.ash.practice.tricky;
import java.util.Collections;
import java.util.LinkedList;
public class CircularLinkedList implements Cloneable{
Node start;
public Node getHead() {
return start;
}
CircularLinkedList setHead(Node startNode) {
start = startNode;
return this;
}
public void add(String name, int age) {
if(name==null) {
System.out.println("name must not be null.");
return;
}
if(start == null) {
Node node = new Node(name,age);
start = node;
node.next = start;
} else {
Node node = new Node(name,age);
Node temp = start;
while(temp.next != start) {
temp = temp.next;
}
temp.next = node;
node.next = start;
}
}
public CircularLinkedList clone()throws CloneNotSupportedException{
return (CircularLinkedList)super.clone();
}
public boolean remove(String name) {
if(name==null) {
return false;
} else if(start==null) {
return false;
} else if(start.getName().equals(name)) {
if(size()>1) {
Node temp = start;
while(temp.next!=start) {
temp = temp.next;
}
temp.next = start.next;
start = start.next;
} else {
start = null;
}
return true;
} else {
Node temp = start;
Node next = null;
Node prev = null;
while(temp.next != start) {
String currName = temp.name;
if(currName.equals(name)) {
next = temp.next;
break;
} else {
temp = temp.next;
}
}
if(next == null) {
return false;
}
prev = temp.next;
while(prev.next!=temp) {
prev = prev.next;
}
prev.next = next;
temp = null;
return true;
}
}
/*
public Node getPrevious(String name, int age) {
Node curr = new Node(name,age);
Node temp = curr;
while(temp.next!=curr) {
temp = temp.next;
}
return temp;
}
*/
public int size() {
int count = 1;
if(start != null) {
Node temp = start;
while(temp.next!=start) {
count++;
temp = temp.next;
}
} else return 0;
return count;
}
public int listSize() {
int count = 1;
if(start != null) {
Node temp = start;
while(temp.next!=null) {
count++;
temp = temp.next;
}
} else return 0;
return count;
}
public void display() {
if(start == null) {
System.out.println("No element present in list.");
} else {
Node temp = start;
while(temp.next != start) {
System.out.println(temp);
temp = temp.next;
}
System.out.println(temp);
}
}
public void displayList() {
if(start == null) {
System.out.println("No element present in list.");
} else {
Node temp = start;
while(temp.next != null) {
System.out.println(temp);
temp = temp.next;
}
System.out.println(temp);
}
}
public Node getPrevious(Node curr) {
if(curr==null) {
return null;
} else {
Node temp = curr;
while(temp.next!=curr) {
temp = temp.next;
}
return temp;
}
}
Node getMiddle() {
Node result = null;
Node temp = start.next;
result = start.next;
Node end = getPrevious(start);
end.next = null;
while(temp.next!=null) {
if(temp.next.next!=null) {
temp = temp.next.next;
result = result.next;
} else {
return result;
}
}
return result;
}
private static CircularLinkedList SortCollections(CircularLinkedList list) {
return SortCollections.doSortBasedOnAge(list);
}
private static class Node {
Node next;
String name;
int age;
Node(String name,int age) {
this.name = name;
this.age = age;
}
String getName() {
return name;
}
int getAge() {
return age;
}
public String toString() {
return "name = "+name +" : age = "+age;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Node other = (Node) obj;
if (age != other.age)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
private static class SortCollections {
static Node mergeSort(Node head) {
if(head == null || head.next == null) {
return head;
}
Node middle = getMiddle(head);
Node nextHead = middle.next;
middle.next = null;
Node left = mergeSort(head);
Node right = mergeSort(nextHead);
Node sortedList = sortedMerged(left, right);
return sortedList;
}
public static CircularLinkedList doSortBasedOnAge(CircularLinkedList list) {
CircularLinkedList copy = null;
try {
copy = list.clone();
} catch (CloneNotSupportedException e) {
e.printStackTrace();
}
if(copy!=null) {
Node head = copy.getHead();
Node end = copy.getPrevious(head);
end.next = null;
Node startNode = mergeSort(head);
CircularLinkedList resultList = new CircularLinkedList().setHead(startNode);
return resultList;
} else {
System.out.println("copy is null");
}
return null;
}
private static Node sortedMerged(Node a, Node b) {
if(a == null) {
return b;
} else if(b == null) {
return a;
}
Node result = null;
if(a.getAge() > b.getAge()) {
result = b;
result.next = sortedMerged(a, b.next);
} else {
result = a;
result.next = sortedMerged(a.next, b);
}
return result;
}
private static Node getMiddle(Node head) {
Node result = null;
Node temp = head;
result = head;
while(temp.next!=null) {
if(temp.next.next!=null) {
temp = temp.next.next;
result = result.next;
} else {
return result;
}
}
return result;
}
}
public static void main(String[] args) {
CircularLinkedList list = new CircularLinkedList();
Collections.sort(new LinkedList());
list.add("ashish", 90);
list.add("rahul", 80);
list.add("deepak", 57);
list.add("ankit", 24);
list.add("raju", 45);
list.add("piyush", 78);
list.add("amit", 12);
//list.display();
/*System.out.println("---------------- size = "+list.size());
System.out.println(list.remove("deepak"));
//list.display();
System.out.println("---------------- size = "+list.size());
System.out.println(list.remove("ashish"));
//list.display();
System.out.println("---------------- size = "+list.size());
System.out.println(list.remove("raju"));
//list.display();
System.out.println("---------------- size = "+list.size());
list.add("aman", 23);
System.out.println("---------------- size = "+list.size());
list.display();
System.out.println("Previous Node of second node is : "+list.getPrevious(list.start.next));
System.out.println("Previous Node of start node is : "+list.getPrevious(list.start));
System.out.println("Previous Node of piyush node is : "+list.getPrevious("piyush",78));*/
list.display();
System.out.println("---------------- size = "+list.size());
//System.out.println(list.getMiddle());
CircularLinkedList newList = CircularLinkedList.SortCollections(list);
newList.displayList();
System.out.println("---------------- size = "+newList.listSize());
}
}

Let's consider the following situation:
The list contains elements B,C,X. Now you want to insert A and then Z.
void insert(int e) {
Node nNode = new Node(); //the new node, step 1: A, step2: Z
Node tNode = head; //step1: points to B, step2: points to A
nNode.element = e;
if (head == null) { //false in both steps
head = nNode;
head.next = head; //I added this line, otherwise you'd never get a circular list
} //don't forget the curly braces when adding more than one statement to a block
else if (head.element > e) { //true in step 1, false in step 2
nNode.next = head; //A.next = A
head=nNode; //A is the new head, but X.next will still be B
} else {
//you'll enter here when adding Z
Node pNode = head; //points to A because of step 1
//when tNode = X you'll start over at B, due to error in step 1
//the condition will never be false, since no node's next will point to A
//and no node's element is greater than Z
while (tNode.next != head && tNode.element <= e) {
pNode = tNode;
tNode = tNode.next;
}
//in contrast to my previous answer, where I had an error in my thought process,
//this is correct: the node is inserted between pNode and tNode
pNode.next = nNode;
nNode.next = tNode;
tNode.next=head; //delete this
}
}
As you can see, there are at least the following problems in your code:
tNode.next=head; is not necessary, since if you insert a node between pNode and tNode, tNode.next should not be affected (and if tNode is the last node, next should already point to the head, while in all other cases this assignment would be wrong).
In the two branches above, where you set head, you're not setting the next element of the last node to head. If you don't do this when adding the first node, that's not necessarily a problem, but leaving that out when adding a new head (second condition) you'll produce an incorrect state which then might result in endless loops
What you might want to do:
Remove the tNode.next=head; statement.
If you add a new head locate the last node and set the head as its next node. That means that if you have only one node, it references itself. If you add a node at the front (your second condition) you'll have to update the next reference of the last node, otherwise you'll get an endless loop if you try to add an element at the end.

After working two days on the code I finally solved it but this is not efficient code .
void insert(int e) {
Node nNode = new Node(); //the new node, step 1: A, step2: Z
Node tNode = head; //step1: points to B, step2: points to A
nNode.element = e;
if (head == null) { //false in both steps
head = nNode;
head.next = head;
}
else if (head.element > e) { //true in step 1, false in step 2
Node pNode = head;
pNode=tNode.next; //PNode is at head which will equal to tNode.next Which will be the next element
nNode.next = head;
head=nNode;
tNode.next.next=nNode; // Now I am moving the Tail Node next
} else {
Node pNode=head; //points to A because of step 1
while (tNode.next != head && tNode.element <= e) {
pNode = tNode;
tNode = tNode.next;
}
pNode.next = nNode;
nNode.next = tNode;
}
}

Erorr with the nullpointer

public class List {
private class Node {
public Node next = null;
Object element;
Node text;
Node(Object element, Node prevNode) {
this.element = element;
prevNode = this;
}
Node(Object element) {
this.element = element;
element = null;
}
}
private Node head;
private Node tail;
private int count;
public List() {
this.head = null;
this.tail = null;
this.count = 0;
}
public void add(Object item) {
if (head == null) {
// We have empty list
head = new Node(item);
tail = head;
} else {
// We have non-empty list
Node newNode = new Node(item, tail);
tail = newNode;
}
count++;
}
public Object remove(int index) {
if (index >= count || index < 0) {
throw new IndexOutOfBoundsException("Invalid index: " + index);
}
// Find the element at the specified index
int currentIndex = 0;
Node currentNode = head;
Node prevNode = null;
while (currentIndex < index) {
prevNode = currentNode;
currentNode = currentNode.next;
currentIndex++;
count--;
if (count == 0) {
head = null;
tail = null;
} else if (prevNode == null) {
head = currentNode.next;
} else {
prevNode.next = currentNode.next;
}
}
return currentNode.element;
}
public int remove(Object item) {
// Find the element containing searched item
int currentIndex = 0;
Node currentNode = head;
Node prevNode = null;
while (currentNode != null) {
if ((currentNode.element != null && currentNode.element
.equals(item))
|| (currentNode.element == null)
&& (item == null)) {
break;
}
prevNode = currentNode;
currentNode = currentNode.next;
currentIndex++;
}
if (currentNode != null) {
// Element is found in the list. Remove it
count--;
if (count == 0) {
head = null;
tail = null;
} else if (prevNode == null) {
head = currentNode.next;
} else {
prevNode.next = currentNode.next;
}
return currentIndex;
} else {
// Element is not found in the list
return -1;
}
}
public int indexOf(Object item) {
int index = 0;
Node current = head;
while (current != null) {
if ((current.element != null && current.element.equals(item))
|| (current.element == null) && (item == null)) {
return index;
}
current = current.next;
index++;
}
return -1;
}
public boolean contains(Object item) {
int index = indexOf(item);
boolean found = (index != -1);
return found;
}
public Object elementAt(int index) {
if (index >= count || index < 0) {
throw new IndexOutOfBoundsException("Invalid index: " + index);
}
Node currentNode = this.head;
for (int i = 0; i < index; i++) {
currentNode = currentNode.next;
}
return currentNode.element;
}
public int getLength() {
return count;
}
public static void main(String[] args) throws NullPointerException {
List shoppingList = new List();
shoppingList.add("Milk");
shoppingList.add("Honey");
shoppingList.add("Olives");
shoppingList.add("Beer");
shoppingList.remove("Olives");
System.out.println("We need to buy:");
for (int i = 0; i < shoppingList.getLength(); i++) {
System.out.println(shoppingList.elementAt(i));
}
System.out.println("Do we have to buy Bread? "
+ shoppingList.contains("Bread"));
}
}
Its giving me that in maina and index of have nullpointer.
Where is my mistake ?
And i want to know how to fix this problem. Thank you.
I would be very grateful if you could give me some feedback for code.

You do not set next when you create your Nodes, so next is always null.
This
Node(final Object element, Node prevNode) {
this.element = element;
prevNode = this;
}
should be
Node(final Object element, Node prevNode) {
this.element = element;
prevNode.next = this;
}

I understand there's something in your code that's giving you nullpointerException when you run the program, but there's no need of it being in the main method, any method called by main may be giving the exception. Still, try this:
Replace this line:
for (int i=0; i<shoppingList.getLength(); i++) {
by this one:
for (int i=0; i < shoppingList.length(); i++) {
if .length() doesn't work, maybe .size() will do.
for (int i=0; i<shoppingList.size(); i++) {
If you are using eclipse, netbeans or any other environment, post which line is throwing the NullPointerException. We can't help you unless you give us more data.

Finding if a number is equal to sum of 2 nodes in a binary search tree

Here is my code that for this. I am traversing the whole tree and then doing a find on each node. find() takes O(log n), and so the whole program takes O(n log n) time.
Is there a better way to implement this program? I am not just talking of better in terms of time complexity but in general as well. How best to implement this?
public boolean searchNum(BinTreeNode node, int num) {
//validate the input
if (node == null) {
return false;
}
// terminal case for recursion
int result = num - node.item;
//I have a separate find() which finds if the key is in the tree
if (find(result)) {
return true;
}
return seachNum(node.leftChild, num) || searchNum(node.rightChilde, num);
}
public boolean find(int key) {
BinTreeNode node = findHelper(key, root);
if (node == null) {
return false;
} else {
return true;
}
}
private BinTreeNode findHelper(int key, BinTreeNode node) {
if (node == null) {
return null;
}
if (key == node.item) {
return node;
} else if (key < node.item) {
return findHelper(key, node.leftChild);
} else {
return findHelper(key, node.rightChild);
}
}

Finding two nodes in binary search tree sum to some value can be done in the similar way of finding two elements in a sorted array that sums to the value.
In the case with an array sorted from small to large, you keep two pointers, one start from beginning, one start from the end. If the sum of the two elements pointed by the pointers is larger than the target, you move the right pointer to left by one, if the sum is smaller than target, you move the left pointer to right by one. Eventually the two pointer will either points to two elements that sum to the target value, or meet in the middle.
boolean searchNumArray(int[] arr, int num) {
int left = 0;
int right = arr.length - 1;
while (left < right) {
int sum = arr[left] + arr[right];
if (sum == num) {
return true;
} else if (sum > num) {
right--;
} else {
left++;
}
}
return false;
}
If you do an in-order traversal of the binary search tree, it becomes a sorted array. So you can apply the same idea on binary search tree.
The following code do iterative in-order traversal from both directions. Stack is being used for the traversal, so the time complexity is O(n) and space complexity is O(h), where h is the height of the binary tree.
class BinTreeIterator implements Iterator<BinTreeNode> {
Stack<BinTreeNode> stack;
boolean leftToRight;
public boolean hasNext() {
return !stack.empty();
}
public BinTreeNode next() {
return stack.peek();
}
public void remove() {
BinTreeNode node = stack.pop();
if (leftToRight) {
node = node.rightChild;
while (node.rightChild != null) {
stack.push(node);
node = node.rightChild;
}
} else {
node = node.leftChild;
while (node.leftChild != null) {
stack.push(node);
node = node.leftChild;
}
}
}
public BinTreeIterator(BinTreeNode node, boolean leftToRight) {
stack = new Stack<BinTreeNode>();
this.leftChildToRight = leftToRight;
if (leftToRight) {
while (node != null) {
stack.push(node);
node = node.leftChild;
}
} else {
while (node != null) {
stack.push(node);
node = node.rightChild;
}
}
}
}
public static boolean searchNumBinTree(BinTreeNode node, int num) {
if (node == null)
return false;
BinTreeIterator leftIter = new BinTreeIterator(node, true);
BinTreeIterator rightIter = new BinTreeIterator(node, false);
while (leftIter.hasNext() && rightIter.hasNext()) {
BinTreeNode left = leftIter.next();
BinTreeNode right = rightIter.next();
int sum = left.item + right.item;
if (sum == num) {
return true;
} else if (sum > num) {
rightIter.remove();
if (!rightIter.hasNext() || rightIter.next() == left) {
return false;
}
} else {
leftIter.remove();
if (!leftIter.hasNext() || leftIter.next() == right) {
return false;
}
}
}
return false;
}

Chen Pang has already given a perfect answer. However, I was trying the same problem today and I could come up with the following solution. Posting it here as it might help some one.
The idea is same as that of earlier solution, just that I am doing it with two stacks - one following the inorder(stack1) and another following reverse - inorder order(stack2). Once we reach the left-most and the right-most node in a BST, we can start comparing them together.
If the sum is less than the required value, pop out from stack1, else pop from stack2. Following is java implementation of the same:
public int sum2(TreeNode A, int B) {
Stack<TreeNode> stack1 = new Stack<>();
Stack<TreeNode> stack2 = new Stack<>();
TreeNode cur1 = A;
TreeNode cur2 = A;
while (!stack1.isEmpty() || !stack2.isEmpty() ||
cur1 != null || cur2 != null) {
if (cur1 != null || cur2 != null) {
if (cur1 != null) {
stack1.push(cur1);
cur1 = cur1.left;
}
if (cur2 != null) {
stack2.push(cur2);
cur2 = cur2.right;
}
} else {
int val1 = stack1.peek().val;
int val2 = stack2.peek().val;
// need to break out of here
if (stack1.peek() == stack2.peek()) break;
if (val1 + val2 == B) return 1;
if (val1 + val2 < B) {
cur1 = stack1.pop();
cur1 = cur1.right;
} else {
cur2 = stack2.pop();
cur2 = cur2.left;
}
}
}
return 0;
}

As far as I know, O(log n) is the best possible searching function you can use. I'm interested in the "n". If you're using a for-loop somewhere, consider using a hashtable in its stead. Hash table seeking is O(1) if I recall correctly.

From http://www.geeksforgeeks.org/find-a-pair-with-given-sum-in-bst/
/* In a balanced binary search tree isPairPresent two element which sums to
a given value time O(n) space O(logn) */
#include <stdio.h>
#include <stdlib.h>
#define MAX_SIZE 100
// A BST node
struct node
{
int val;
struct node *left, *right;
};
// Stack type
struct Stack
{
int size;
int top;
struct node* *array;
};
// A utility function to create a stack of given size
struct Stack* createStack(int size)
{
struct Stack* stack =
(struct Stack*) malloc(sizeof(struct Stack));
stack->size = size;
stack->top = -1;
stack->array =
(struct node**) malloc(stack->size * sizeof(struct node*));
return stack;
}
// BASIC OPERATIONS OF STACK
int isFull(struct Stack* stack)
{ return stack->top - 1 == stack->size; }
int isEmpty(struct Stack* stack)
{ return stack->top == -1; }
void push(struct Stack* stack, struct node* node)
{
if (isFull(stack))
return;
stack->array[++stack->top] = node;
}
struct node* pop(struct Stack* stack)
{
if (isEmpty(stack))
return NULL;
return stack->array[stack->top--];
}
// Returns true if a pair with target sum exists in BST, otherwise false
bool isPairPresent(struct node *root, int target)
{
// Create two stacks. s1 is used for normal inorder traversal
// and s2 is used for reverse inorder traversal
struct Stack* s1 = createStack(MAX_SIZE);
struct Stack* s2 = createStack(MAX_SIZE);
// Note the sizes of stacks is MAX_SIZE, we can find the tree size and
// fix stack size as O(Logn) for balanced trees like AVL and Red Black
// tree. We have used MAX_SIZE to keep the code simple
// done1, val1 and curr1 are used for normal inorder traversal using s1
// done2, val2 and curr2 are used for reverse inorder traversal using s2
bool done1 = false, done2 = false;
int val1 = 0, val2 = 0;
struct node *curr1 = root, *curr2 = root;
// The loop will break when we either find a pair or one of the two
// traversals is complete
while (1)
{
// Find next node in normal Inorder traversal. See following post
// http://www.geeksforgeeks.org/inorder-tree-traversal-without-recursion/
while (done1 == false)
{
if (curr1 != NULL)
{
push(s1, curr1);
curr1 = curr1->left;
}
else
{
if (isEmpty(s1))
done1 = 1;
else
{
curr1 = pop(s1);
val1 = curr1->val;
curr1 = curr1->right;
done1 = 1;
}
}
}
// Find next node in REVERSE Inorder traversal. The only
// difference between above and below loop is, in below loop
// right subtree is traversed before left subtree
while (done2 == false)
{
if (curr2 != NULL)
{
push(s2, curr2);
curr2 = curr2->right;
}
else
{
if (isEmpty(s2))
done2 = 1;
else
{
curr2 = pop(s2);
val2 = curr2->val;
curr2 = curr2->left;
done2 = 1;
}
}
}
// If we find a pair, then print the pair and return. The first
// condition makes sure that two same values are not added
if ((val1 != val2) && (val1 + val2) == target)
{
printf("\n Pair Found: %d + %d = %d\n", val1, val2, target);
return true;
}
// If sum of current values is smaller, then move to next node in
// normal inorder traversal
else if ((val1 + val2) < target)
done1 = false;
// If sum of current values is greater, then move to next node in
// reverse inorder traversal
else if ((val1 + val2) > target)
done2 = false;
// If any of the inorder traversals is over, then there is no pair
// so return false
if (val1 >= val2)
return false;
}
}
// A utility function to create BST node
struct node * NewNode(int val)
{
struct node *tmp = (struct node *)malloc(sizeof(struct node));
tmp->val = val;
tmp->right = tmp->left =NULL;
return tmp;
}
// Driver program to test above functions
int main()
{
/*
15
/ \
10 20
/ \ / \
8 12 16 25 */
struct node *root = NewNode(15);
root->left = NewNode(10);
root->right = NewNode(20);
root->left->left = NewNode(8);
root->left->right = NewNode(12);
root->right->left = NewNode(16);
root->right->right = NewNode(25);
int target = 28;
if (isPairPresent(root, target) == false)
printf("\n No such values are found\n");
getchar();
return 0;
}

I found one bug under Chen Pang answer otherwise it's perfect. Bug is under remove method. All other code is same except remove method under iterator
suppose we have tree, per Chen Pang answer its will not consider element 18. similarly for left iterator
10
20
15 25
13 18
class BinTreeIterator implements Iterator<BinTreeNode> {
Stack<BinTreeNode> stack;
boolean leftToRight;
public boolean hasNext() {
return !stack.empty();
}
public BinTreeNode next() {
return stack.peek();
}
public void remove() {
BinTreeNode node = stack.pop();
if (leftToRight) {
node = node.rightChild;
while (node.rightChild != null) {
stack.push(node);
BinTreeNode leftNode=node.leftChild;
while (leftNode != null) {
stack.push(leftNode);
leftNode= node.leftChild;
}
node = node.rightChild;
}
} else {
node = node.leftChild;
while (node.leftChild != null) {
stack.push(node);
BinTreeNode rightNode=node.rightChild;
while (rightNode != null) {
stack.push(rightNode);
rightNode= node.rightChild;
}
node = node.leftChild;
}
}
}
public BinTreeIterator(BinTreeNode node, boolean leftToRight) {
stack = new Stack<BinTreeNode>();
this.leftToRight = leftToRight;
if (leftToRight) {
while (node != null) {
stack.push(node);
node = node.leftChild;
}
} else {
while (node != null) {
stack.push(node);
node = node.rightChild;
}
}
}
public static boolean searchNumBinTree(BinTreeNode node, int num) {
if (node == null)
return false;
BinTreeIterator leftIter = new BinTreeIterator(node,true);
BinTreeIterator rightIter = new BinTreeIterator(node,false);
while (leftIter.hasNext() && rightIter.hasNext()) {
BinTreeNode left = leftIter.next();
BinTreeNode right = rightIter.next();
int sum = left.item + right.item;
if (sum == num) {
return true;
} else if (sum > num) {
rightIter.remove();
if (!rightIter.hasNext() || rightIter.next() == left) {
return false;
}
} else {
leftIter.remove();
if (!leftIter.hasNext() || leftIter.next() == right) {
return false;
}
}
}
return false;
}
private static class BinTreeNode{
BinTreeNode leftChild;
BinTreeNode rightChild;
}
}

public boolean nodeSum(Node root, int num){
/*
Just subtract the sum from current value of the node and find
the node with remainder.
*/
if(root==null){
return false;
}
int val=num-root.key;
boolean found=find(root,val);
if(found){
return true;
}
boolean lSum=nodeSum(root.left,num);
boolean rSum=nodeSum(root.right,num);
return lSum||rSum;
}
public boolean find(Node root, int k){//same as search
if(root==null){
return false;
}
if(root.key==k){
return true;
}
if(root.key<k){
return find(root.right,k);
}else{
return find(root.left, k);
}
}