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isBST

Language/Type: C++ binary trees pointers recursion

Write a function named isBST that accepts a pointer to the root of a binary tree of integers. Your function should return whether or not a binary tree is arranged in valid binary search tree (BST) order. A BST is a tree in which every node N's left subtree is a BST that contains only values less than N's data, and its right subtree is a BST that contains only values greater than N's data. The empty (nullptr) tree is a BST by definition.

Constraints: You must implement your function recursively and without using loops. Do not construct any new BinaryTreeNode objects in solving this problem (though you may create as many BinaryTreeNode* pointer variables as you like). Do not use any auxiliary data structures to solve this problem (no array, vector, stack, queue, string, etc). Your function should not modify the tree's state; the state of the tree should remain constant with respect to your function.

Assume that you are using the BinaryTreeNode structure as defined below: 

struct BinaryTreeNode {
    int data;
    BinaryTreeNode* left;
    BinaryTreeNode* right;
};
Function: Write a C++ function as described, not a complete program.

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