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