Write a function that returns **true** if the given Binary Tree of size **N** is SumTree else return **false**. A SumTree is a Binary Tree in which value of each node x is equal to sum of nodes present in its left subtree and right subtree . An empty tree is SumTree and sum of an empty tree can be considered as 0. A leaf node is also considered as SumTree.

Following is an example of SumTree.

```
26
/ \
10 3
/ \ / \
4 6 1 2
```