Difference between sums of odd level and even level 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 level and even level. Assume root at level 1, left/right child of root at level 2 and so on.

Example

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

Sum of nodes at odd level
= 5 + 1 + 4 + 8
= 18
Sum of nodes at even level
= 2 + 6 + 3 + 7 + 9
= 27
Difference = -9.

Solution

Use Recursive Traversal. During traversal, return the difference of root node and its left and right child.

Example

 Live Demo

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

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(Node node){
      if(node == null) return 0;
      return node.data - difference(node.left) - difference(node.right);
   }
   public static void main(String args[]){
      Node tree = getTree();
      System.out.println(difference(tree));
   }
}

Output

-9