Timer is Running

Given a binary tree. Check whether it is a BST or not.

Input Format:

  1. The values in the string are in the order of level order traversal of the tree where, numbers denote node values, and a character “N” denotes NULL child.

  2. For example:
    For the above tree, the string will be: 1 2 3 N N 4 6 N 5 N N 7 N

Example 1:

S = 2 1 3
Output: 1
Explanation: The given Binary Tree is:
 /    \ 
1      3
This is clearly a BST. Hence the output
is 1.

Example 2:

S = 2 N 7 N 6 N 5 N 9 N 2 N 6
Output: 0

Your Task:
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

Note: The Input/Output format and Example are given are used for the system's internal purpose, and should be used by a user for Expected Output only. As it is a function problem, hence a user should not read any input from the stdin/console. The task is to complete the function specified, and not to write the full code.

** For More Input/Output Examples Use 'Expected Output' option **

Contributor: Amit Khandelwal, Harshit Sidhwa
Author: kartik

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