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Given a binary tree. Find the size of its largest subtree that is a Binary Search Tree.

**Note: **Here Size is equal to the number of nodes in the subtree.

**Example 1:**

**Input:**
1
/ \
4 4
/ \
6 8**
Output: **1**
Explanation: **There's no sub-tree with size
greater than 1 which forms a BST. All the
leaf Nodes are the BSTs with size equal
to 1.

**Example 2:**

**Input: **6 6 3 N 2 9 3 N 8 8 2
6
/ \
6 3
\ / \
2 9 3
\ / \
8 8 2 **
Output: **2**
Explanation: **The following sub-tree is a
BST of size 2:
2
/ \
N 8

**Your Task:**

You don't need to read input or print anything. Your task is to complete the function** largestBst() **that takes the root node of the Binary Tree** **as its input and returns the size of the largest subtree which is also the BST. If the complete Binary Tree is a BST, return the size of the complete Binary Tree.

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

**Expected Auxiliary Space: **O(Height of the BST).

**Constraints:**

1 ≤ Number of nodes ≤ 10^{5}

1 ≤ Data of a node ≤ 10^{6}

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Largest BST

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