Difference between sums of odd position and even position nodes of a Binary Tree in Java

Problem Statement

With a given binary tree, write a program to find the difference between sum of nodes at odd position and even position. Assume root at level 0, odd position, left/right child of root at level 2, left child at odd position and right child at even position and so on.

Example

    5
   / \
  2   6
 / \   \
 1 4    8
   /    / \
   3    7  9

Sum of nodes at odd positions
= 5 + 2 + (1 + 8) + (3 + 9)
= 5 + 2 + 9 + 12
= 28
Sum of nodes at even level
= 0 + 6 + 4 + 7
= 17
Difference = 11.

Solution

Use Level Order Traversal. During traversal, mark first element as odd position and then switch to even when new element is encountered and then switch back on next and so on.

Example

 Live Demo

Following is the program in Java to find the required output.

import java.util.LinkedList;

class Node {
   int data;
   Node left, right;
   Node(int data){
      this.data = data;
      this.left = this.right = null;
   }
}

public class JavaTester {
   public static Node getTree(){
      Node root = new Node(5);
      root.left = new Node(2);
      root.right = new Node(6);
      root.left.left = new Node(1);
      root.left.right = new Node(4);
      root.left.right.left = new Node(3);
      root.right.right = new Node(8);
      root.right.right.right = new Node(9);
      root.right.right.left = new Node(7);
      return root;
   }
   public static int difference(LinkedList<Node> queue){
      if(queue.isEmpty()) return 0;
      int evenSum = 0;
      int oddSum = 0;

      while(true){
         int nodes = queue.size();
         if(nodes == 0) break;
         boolean isOdd = true;
         while(nodes > 0){
            Node node = queue.peek();
            if(isOdd) oddSum += node.data;
            else evenSum += node.data;
            queue.remove();
            nodes--;
            if(node.left != null) queue.add(node.left);
            if(node.right != null) queue.add(node.right);
            isOdd = !isOdd;
         }
      }
      return oddSum - evenSum;
   }

   public static void main(String args[]){
      Node tree = getTree();
      LinkedList<Node> queue = new LinkedList<Node>();
      queue.add(tree);
      System.out.println(difference(queue));
   }
}

Output

11