Java Program to calculate the Difference between the Sum of the Odd Level and the Even Level Nodes of a Binary Tree

import java.util.LinkedList;    
import java.util.Queue;    
     
public class DiffOddEven {    
        
    //Represent a node of binary tree    
    public static class Node{    
        int data;    
        Node left;    
        Node right;    
            
        public Node(int data){    
            //Assign data to the new node, set left and right children to null    
            this.data = data;    
            this.left = null;    
            this.right = null;    
            }    
        }    
        
    //Represent the root of binary tree    
    public Node root;    
      
    public DiffOddEven(){    
        root = null;    
    }    
     
    //difference() will calculate the difference between sum of odd and even levels of binary tree    
    public int difference() {    
          int oddLevel = 0, evenLevel = 0, diffOddEven = 0;    
              
          //Variable nodesInLevel keep tracks of number of nodes in each level    
          int nodesInLevel = 0;    
              
          //Variable currentLevel keep track of level in binary tree    
          int currentLevel = 0;    
              
          //Queue will be used to keep track of nodes of tree level-wise    
          Queue<Node> queue = new LinkedList<Node>();    
              
          //Check if root is null    
          if(root == null) {    
              System.out.println("Tree is empty");    
              return 0;    
          }    
          else {    
              //Add root node to queue as it represents the first level    
              queue.add(root);    
              currentLevel++;    
                  
              while(queue.size() != 0) {    
                      
                  //Variable nodesInLevel will hold the size of queue i.e. number of elements in queue    
                  nodesInLevel = queue.size();    
                      
                  while(nodesInLevel > 0) {    
                      Node current = queue.remove();    
                          
                    //Checks if currentLevel is even or not.    
                      if(currentLevel % 2 == 0)    
                          //If level is even, add nodes's to variable evenLevel    
                          evenLevel += current.data;    
                      else    
                          //If level is odd, add nodes's to variable oddLevel    
                          oddLevel += current.data;    
                          
                      //Adds left child to queue    
                      if(current.left != null)    
                          queue.add(current.left);    
                      //Adds right child to queue    
                      if(current.right != null)     
                          queue.add(current.right);    
                     nodesInLevel--;    
                  }    
                  currentLevel++;    
              }    
              //Calculates difference between oddLevel and evenLevel    
              diffOddEven = Math.abs(oddLevel - evenLevel);    
          }    
          return diffOddEven;    
      }    
      
    public static void main (String[] args) {    
            
        DiffOddEven bt = new DiffOddEven();    
        //Add nodes to the binary tree    
        bt.root = new Node(1);    
        bt.root.left = new Node(2);    
        bt.root.right = new Node(3);    
        bt.root.left.left = new Node(4);    
        bt.root.right.left = new Node(5);    
        bt.root.right.right = new Node(6);    
        
        //Display the difference between sum of odd level and even level nodes    
        System.out.println("Difference between sum of odd level and even level nodes: " + bt.difference());    
}    
}   

Output:

Difference between sum of odd level and even level nodes: 11