Given the root of a binary tree. Check whether it is a BST or not.
Note: We are considering that BSTs can not contain duplicate Nodes.
A BST is defined as follows:
Input: 2 / \ 1 3 Output: 1 Explanation: The left subtree of root node contains node with key lesser than the root node’s key and the right subtree of root node contains node with key greater than the root node’s key. Hence, the tree is a BST.
Input: 2 \ 7 \ 6 \ 5 \ 9 \ 2 \ 6 Output: 0 Explanation: Since the node with value 7 has right subtree nodes with keys less than 7, this is not a BST.
You don't need to read input or print anything. Your task is to complete the function isBST() which takes the root of the tree as a parameter and returns true if the given binary tree is BST, else returns false.
Expected Time Complexity: O(N).
Expected Auxiliary Space: O(Height of the BST).
0 <= Number of edges <= 100000
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