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

Example 1:

Input:
    2
 /    \
1      3
Output: 1

Example 2:

Input:
  2
   \
    7
     \
      6
       \
        5
         \
          9
           \
            2
             \
              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. The printing is done by the driver's code.

Expected Time Complexity: O(N).
Expected Auxiliary Space: O(Height of the BST).

Constraints:
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.