Find the Sum of all Elements in a Binary Tree

Given a binary tree, write a method to find and return the sum of all the elements using recursion. For an empty tree the sum is 0.

Example:  
       1
      / \
     2   3    
    / \ / \        
   4  5 6  7
  /
 8   
  ==> sum of all nodes = 36 
  (1+2+3+4+5+6+7+8)

Need a hand? Try out these hints, one at a time.

Sum up the data of the nodes recursively.

1. If root is null return 0

2. Else return root.data + sum(root.left) + sum(root.right))

public int sum(TreeNode root) { 
    



}

Java

Python