Given a Binary Tree. Check whether it is a Sum Tree or not.

A Binary Tree is a Sum 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 also a Sum Tree as sum of an empty tree can be considered to be 0. A leaf node is also considered as a Sum Tree.

**Example 1:**

Input:3 / \ 1 2Output:1Explanation:The given tree is a sum tree so return a boolean true.

**Example 2:**

Input:10 / \ 20 30 / \ 10 10Output:0Explanation:The given tree is not a sum tree. For the root node, sum of elements in left subtree is 40 and sum of elements in right subtree is 30. Root element = 10 which is not equal to 30+40.

**Your Task: **

You dont need to read input or print anything. Complete the function **isSumTree() **which takes root node as input parameter and returns true if the tree is a SumTree else it returns false.

**Expected Time Complexity: **O(N)

**Expected Auxiliary Space:** O(height of tree)

**Constraints:**

1 ≤ number of nodes ≤ 10^{4}