# A program to check if a binary tree is BST or not

A binary search tree (BST) is a node based binary tree data structure which has the following properties.
• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

From the above properties it naturally follows that:
• Each node (item in the tree) has a distinct key. METHOD 1 (Simple but Wrong)
Following is a simple program. For each node, check if left node of it is smaller than the node and right node of it is greater than the node.

 `int` `isBST(``struct` `node* node)  ` `{  ` `  ``if` `(node == NULL)  ` `    ``return` `1;  ` `     `  `  ``/* false if left is > than node */` `  ``if` `(node->left != NULL && node->left->data > node->data)  ` `    ``return` `0;  ` `     `  `  ``/* false if right is < than node */` `  ``if` `(node->right != NULL && node->right->data < node->data)  ` `    ``return` `0;  ` `   `  `  ``/* false if, recursively, the left or right is not a BST */` `  ``if` `(!isBST(node->left) || !isBST(node->right))  ` `    ``return` `0;  ` `     `  `  ``/* passing all that, it's a BST */` `  ``return` `1;  ` `} `

This approach is wrong as this will return true for below binary tree (and below tree is not a BST because 4 is in left subtree of 3) METHOD 2 (Correct but not efficient)
For each node, check if max value in left subtree is smaller than the node and min value in right subtree greater than the node.

 `/* Returns true if a binary tree is a binary search tree */`  `int` `isBST(``struct` `node* node)  ` `{  ` `  ``if` `(node == NULL)  ` `    ``return``(``true``);  ` `     `  `  ``/* false if the max of the left is > than us */` `  ``if` `(node->left!=NULL && maxValue(node->left) > node->data)  ` `    ``return``(``false``);  ` `     `  `  ``/* false if the min of the right is <= than us */` `  ``if` `(node->right!=NULL && minValue(node->right) < node->data)  ` `    ``return``(``false``);  ` `   `  `  ``/* false if, recursively, the left or right is not a BST */` `  ``if` `(!isBST(node->left) || !isBST(node->right))  ` `    ``return``(``false``);  ` `     `  `  ``/* passing all that, it's a BST */` `  ``return``(``true``);  ` `}  `

It is assumed that you have helper functions minValue() and maxValue() that return the min or max int value from a non-empty tree

METHOD 3 (Correct and Efficient)
Method 2 above runs slowly since it traverses over some parts of the tree many times. A better solution looks at each node only once. The trick is to write a utility helper function isBSTUtil(struct node* node, int min, int max) that traverses down the tree keeping track of the narrowing min and max allowed values as it goes, looking at each node only once. The initial values for min and max should be INT_MIN and INT_MAX — they narrow from there.

```/* Returns true if the given tree is a binary search tree
(efficient version). */
int isBST(struct node* node)
{
return(isBSTUtil(node, INT_MIN, INT_MAX));
}

/* Returns true if the given tree is a BST and its
values are >= min and <= max. */
int isBSTUtil(struct node* node, int min, int max)
```

Implementation:

## C++

 `#include ` `#include ` `using` `namespace` `std; ` ` `  `/* A binary tree node has data,  ` `pointer to left child and  ` `a pointer to right child */` `class` `node  ` `{  ` `    ``public``: ` `    ``int` `data;  ` `    ``node* left;  ` `    ``node* right;  ` `     `  `    ``/* Constructor that allocates  ` `    ``a new node with the given data ` `    ``and NULL left and right pointers. */` `    ``node(``int` `data) ` `    ``{ ` `        ``this``->data = data; ` `        ``this``->left = NULL; ` `        ``this``->right = NULL; ` `    ``} ` `}; ` ` `  `int` `isBSTUtil(node* node, ``int` `min, ``int` `max);  ` ` `  `/* Returns true if the given  ` `tree is a binary search tree  ` `(efficient version). */` `int` `isBST(node* node)  ` `{  ` `    ``return``(isBSTUtil(node, INT_MIN, INT_MAX));  ` `}  ` ` `  `/* Returns true if the given ` `tree is a BST and its values ` `are >= min and <= max. */` `int` `isBSTUtil(node* node, ``int` `min, ``int` `max)  ` `{  ` `    ``/* an empty tree is BST */` `    ``if` `(node==NULL)  ` `        ``return` `1;  ` `             `  `    ``/* false if this node violates ` `    ``the min/max constraint */` `    ``if` `(node->data < min || node->data > max)  ` `        ``return` `0;  ` `     `  `    ``/* otherwise check the subtrees recursively,  ` `    ``tightening the min or max constraint */` `    ``return` `        ``isBSTUtil(node->left, min, node->data-1) && ``// Allow only distinct values  ` `        ``isBSTUtil(node->right, node->data+1, max); ``// Allow only distinct values  ` `}  ` ` `  ` `  `/* Driver code*/` `int` `main()  ` `{  ` `    ``node *root = ``new` `node(4);  ` `    ``root->left = ``new` `node(2);  ` `    ``root->right = ``new` `node(5);  ` `    ``root->left->left = ``new` `node(1);  ` `    ``root->left->right = ``new` `node(3);  ` `     `  `    ``if``(isBST(root))  ` `        ``cout<<``"Is BST"``;  ` `    ``else` `        ``cout<<``"Not a BST"``;  ` `         `  `    ``return` `0;  ` `}  ` ` `  `// This code is contributed by rathbhupendra `

## C

 `#include ` `#include ` `#include ` ` `  `/* A binary tree node has data, pointer to left child ` `   ``and a pointer to right child */` `struct` `node ` `{ ` `    ``int` `data; ` `    ``struct` `node* left; ` `    ``struct` `node* right; ` `}; ` ` `  `int` `isBSTUtil(``struct` `node* node, ``int` `min, ``int` `max); ` ` `  `/* Returns true if the given tree is a binary search tree  ` ` ``(efficient version). */`  `int` `isBST(``struct` `node* node)  ` `{  ` `  ``return``(isBSTUtil(node, INT_MIN, INT_MAX));  ` `}  ` ` `  `/* Returns true if the given tree is a BST and its  ` `   ``values are >= min and <= max. */`  `int` `isBSTUtil(``struct` `node* node, ``int` `min, ``int` `max)  ` `{  ` `  ``/* an empty tree is BST */` `  ``if` `(node==NULL)  ` `     ``return` `1; ` `       `  `  ``/* false if this node violates the min/max constraint */`   `  ``if` `(node->data < min || node->data > max)  ` `     ``return` `0;  ` ` `  `  ``/* otherwise check the subtrees recursively,  ` `   ``tightening the min or max constraint */` `  ``return`  `    ``isBSTUtil(node->left, min, node->data-1) &&  ``// Allow only distinct values ` `    ``isBSTUtil(node->right, node->data+1, max);  ``// Allow only distinct values ` `}  ` ` `  `/* Helper function that allocates a new node with the ` `   ``given data and NULL left and right pointers. */` `struct` `node* newNode(``int` `data) ` `{ ` `  ``struct` `node* node = (``struct` `node*) ` `                       ``malloc``(``sizeof``(``struct` `node)); ` `  ``node->data = data; ` `  ``node->left = NULL; ` `  ``node->right = NULL; ` ` `  `  ``return``(node); ` `} ` ` `  `/* Driver program to test above functions*/` `int` `main() ` `{ ` `  ``struct` `node *root = newNode(4); ` `  ``root->left        = newNode(2); ` `  ``root->right       = newNode(5); ` `  ``root->left->left  = newNode(1); ` `  ``root->left->right = newNode(3);  ` ` `  `  ``if``(isBST(root)) ` `    ``printf``(``"Is BST"``); ` `  ``else` `    ``printf``(``"Not a BST"``); ` `     `  `  ``getchar``(); ` `  ``return` `0; ` `}   `

## Java

 `//Java implementation to check if given Binary tree ` `//is a BST or not ` ` `  `/* Class containing left and right child of current ` ` ``node and key value*/` `class` `Node ` `{ ` `    ``int` `data; ` `    ``Node left, right; ` ` `  `    ``public` `Node(``int` `item) ` `    ``{ ` `        ``data = item; ` `        ``left = right = ``null``; ` `    ``} ` `} ` ` `  `public` `class` `BinaryTree ` `{ ` `    ``//Root of the Binary Tree ` `    ``Node root; ` ` `  `    ``/* can give min and max value according to your code or ` `    ``can write a function to find min and max value of tree. */` ` `  `    ``/* returns true if given search tree is binary ` `     ``search tree (efficient version) */` `    ``boolean` `isBST()  { ` `        ``return` `isBSTUtil(root, Integer.MIN_VALUE, ` `                               ``Integer.MAX_VALUE); ` `    ``} ` ` `  `    ``/* Returns true if the given tree is a BST and its ` `      ``values are >= min and <= max. */` `    ``boolean` `isBSTUtil(Node node, ``int` `min, ``int` `max) ` `    ``{ ` `        ``/* an empty tree is BST */` `        ``if` `(node == ``null``) ` `            ``return` `true``; ` ` `  `        ``/* false if this node violates the min/max constraints */` `        ``if` `(node.data < min || node.data > max) ` `            ``return` `false``; ` ` `  `        ``/* otherwise check the subtrees recursively ` `        ``tightening the min/max constraints */` `        ``// Allow only distinct values ` `        ``return` `(isBSTUtil(node.left, min, node.data-``1``) && ` `                ``isBSTUtil(node.right, node.data+``1``, max)); ` `    ``} ` ` `  `    ``/* Driver program to test above functions */` `    ``public` `static` `void` `main(String args[]) ` `    ``{ ` `        ``BinaryTree tree = ``new` `BinaryTree(); ` `        ``tree.root = ``new` `Node(``4``); ` `        ``tree.root.left = ``new` `Node(``2``); ` `        ``tree.root.right = ``new` `Node(``5``); ` `        ``tree.root.left.left = ``new` `Node(``1``); ` `        ``tree.root.left.right = ``new` `Node(``3``); ` ` `  `        ``if` `(tree.isBST()) ` `            ``System.out.println(``"IS BST"``); ` `        ``else` `            ``System.out.println(``"Not a BST"``); ` `    ``} ` `} `

## Python

 `# Python program to check if a binary tree is bst or not ` ` `  `INT_MAX ``=` `4294967296` `INT_MIN ``=` `-``4294967296` ` `  `# A binary tree node ` `class` `Node: ` ` `  `    ``# Constructor to create a new node ` `    ``def` `__init__(``self``, data): ` `        ``self``.data ``=` `data  ` `        ``self``.left ``=` `None` `        ``self``.right ``=` `None` ` `  ` `  `# Returns true if the given tree is a binary search tree ` `# (efficient version) ` `def` `isBST(node): ` `    ``return` `(isBSTUtil(node, INT_MIN, INT_MAX)) ` ` `  `# Retusn true if the given tree is a BST and its values ` `# >= min and <= max ` `def` `isBSTUtil(node, mini, maxi): ` `     `  `    ``# An empty tree is BST ` `    ``if` `node ``is` `None``: ` `        ``return` `True` ` `  `    ``# False if this node violates min/max constraint ` `    ``if` `node.data < mini ``or` `node.data > maxi: ` `        ``return` `False` ` `  `    ``# Otherwise check the subtrees recursively ` `    ``# tightening the min or max constraint ` `    ``return` `(isBSTUtil(node.left, mini, node.data ``-``1``) ``and` `          ``isBSTUtil(node.right, node.data``+``1``, maxi)) ` ` `  `# Driver program to test above function ` `root ``=` `Node(``4``) ` `root.left ``=` `Node(``2``) ` `root.right ``=` `Node(``5``) ` `root.left.left ``=` `Node(``1``) ` `root.left.right ``=` `Node(``3``) ` ` `  `if` `(isBST(root)): ` `    ``print` `"Is BST"` `else``: ` `    ``print` `"Not a BST"` ` `  `# This code is contributed by Nikhil Kumar Singh(nickzuck_007) `

## C#

 `using` `System; ` ` `  `// C# implementation to check if given Binary tree  ` `//is a BST or not  ` ` `  `/* Class containing left and right child of current  ` ` ``node and key value*/` `public` `class` `Node ` `{ ` `    ``public` `int` `data; ` `    ``public` `Node left, right; ` ` `  `    ``public` `Node(``int` `item) ` `    ``{ ` `        ``data = item; ` `        ``left = right = ``null``; ` `    ``} ` `} ` ` `  `public` `class` `BinaryTree ` `{ ` `    ``//Root of the Binary Tree  ` `    ``public` `Node root; ` ` `  `    ``/* can give min and max value according to your code or  ` `    ``can write a function to find min and max value of tree. */` ` `  `    ``/* returns true if given search tree is binary  ` `     ``search tree (efficient version) */` `    ``public` `virtual` `bool` `BST ` `    ``{ ` `        ``get` `        ``{ ` `            ``return` `isBSTUtil(root, ``int``.MinValue, ``int``.MaxValue); ` `        ``} ` `    ``} ` ` `  `    ``/* Returns true if the given tree is a BST and its  ` `      ``values are >= min and <= max. */` `    ``public` `virtual` `bool` `isBSTUtil(Node node, ``int` `min, ``int` `max) ` `    ``{ ` `        ``/* an empty tree is BST */` `        ``if` `(node == ``null``) ` `        ``{ ` `            ``return` `true``; ` `        ``} ` ` `  `        ``/* false if this node violates the min/max constraints */` `        ``if` `(node.data < min || node.data > max) ` `        ``{ ` `            ``return` `false``; ` `        ``} ` ` `  `        ``/* otherwise check the subtrees recursively  ` `        ``tightening the min/max constraints */` `        ``// Allow only distinct values  ` `        ``return` `(isBSTUtil(node.left, min, node.data - 1) && isBSTUtil(node.right, node.data + 1, max)); ` `    ``} ` ` `  `    ``/* Driver program to test above functions */` `    ``public` `static` `void` `Main(``string``[] args) ` `    ``{ ` `        ``BinaryTree tree = ``new` `BinaryTree(); ` `        ``tree.root = ``new` `Node(4); ` `        ``tree.root.left = ``new` `Node(2); ` `        ``tree.root.right = ``new` `Node(5); ` `        ``tree.root.left.left = ``new` `Node(1); ` `        ``tree.root.left.right = ``new` `Node(3); ` ` `  `        ``if` `(tree.BST) ` `        ``{ ` `            ``Console.WriteLine(``"IS BST"``); ` `        ``} ` `        ``else` `        ``{ ` `            ``Console.WriteLine(``"Not a BST"``); ` `        ``} ` `    ``} ` `} ` ` `  `  ``// This code is contributed by Shrikant13 `

Output:

`IS BST`

Time Complexity: O(n)
Auxiliary Space : O(1) if Function Call Stack size is not considered, otherwise O(n)

Simplified Method 3
We can simplify method 2 using NULL pointers instead of INT_MIN and INT_MAX values.

## C++

 `// C++ program to check if a given tree is BST. ` `#include ` `using` `namespace` `std; ` ` `  `/* A binary tree node has data, pointer to ` `   ``left child and a pointer to right child */` `struct` `Node ` `{ ` `    ``int` `data; ` `    ``struct` `Node* left, *right; ` `}; ` ` `  `// Returns true if given tree is BST. ` `bool` `isBST(Node* root, Node* l=NULL, Node* r=NULL) ` `{ ` `    ``// Base condition ` `    ``if` `(root == NULL) ` `        ``return` `true``; ` ` `  `    ``// if left node exist then check it has ` `    ``// correct data or not i.e. left node's data ` `    ``// should be less than root's data ` `    ``if` `(l != NULL and root->data < l->data) ` `        ``return` `false``; ` ` `  `    ``// if right node exist then check it has ` `    ``// correct data or not i.e. right node's data ` `    ``// should be greater than root's data ` `    ``if` `(r != NULL and root->data > r->data) ` `        ``return` `false``; ` ` `  `    ``// check recursively for every node. ` `    ``return` `isBST(root->left, l, root) and ` `           ``isBST(root->right, root, r); ` `} ` ` `  `/* Helper function that allocates a new node with the ` `   ``given data and NULL left and right pointers. */` `struct` `Node* newNode(``int` `data) ` `{ ` `    ``struct` `Node* node = ``new` `Node; ` `    ``node->data = data; ` `    ``node->left = node->right = NULL; ` `    ``return` `(node); ` `} ` ` `  `/* Driver program to test above functions*/` `int` `main() ` `{ ` `    ``struct` `Node *root = newNode(3); ` `    ``root->left        = newNode(2); ` `    ``root->right       = newNode(5); ` `    ``root->left->left  = newNode(1); ` `    ``root->left->right = newNode(4); ` ` `  `    ``if` `(isBST(root,NULL,NULL)) ` `        ``cout << ``"Is BST"``; ` `    ``else` `        ``cout << ``"Not a BST"``; ` ` `  `    ``return` `0; ` `} `

## Python3

 `""" Program to check if a given Binary ` `Tree is balanced like a Red-Black Tree """` ` `  `# Helper function that allocates a new  ` `# node with the given data and None  ` `# left and right poers.                                  ` `class` `newNode:  ` ` `  `    ``# Construct to create a new node  ` `    ``def` `__init__(``self``, key):  ` `        ``self``.data ``=` `key ` `        ``self``.left ``=` `None` `        ``self``.right ``=` `None` ` `  `# Returns true if given tree is BST.  ` `def` `isBST(root, l ``=` `None``, r ``=` `None``):  ` ` `  `    ``# Base condition  ` `    ``if` `(root ``=``=` `None``) : ` `        ``return` `True` ` `  `    ``# if left node exist then check it has  ` `    ``# correct data or not i.e. left node's data  ` `    ``# should be less than root's data  ` `    ``if` `(l !``=` `None` `and` `root.data < l.data) : ` `        ``return` `False` ` `  `    ``# if right node exist then check it has  ` `    ``# correct data or not i.e. right node's data  ` `    ``# should be greater than root's data  ` `    ``if` `(r !``=` `None` `and` `root.data > r.data) : ` `        ``return` `False` ` `  `    ``# check recursively for every node.  ` `    ``return` `isBST(root.left, l, root) ``and` ` ` `        ``isBST(root.right, root, r)  ` ` `  ` `  `# Driver Code  ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``root ``=` `newNode(``3``)  ` `    ``root.left ``=` `newNode(``2``)  ` `    ``root.right ``=` `newNode(``5``)  ` `    ``root.right.left ``=` `newNode(``1``)  ` `    ``root.right.right ``=` `newNode(``4``)  ` `    ``#root.right.left.left = newNode(40) ` `    ``if` `(isBST(root,``None``,``None``)): ` `        ``print``(``"Is BST"``) ` `    ``else``: ` `        ``print``(``"Not a BST"``) ` ` `  `# This code is contributed by ` `# Shubham Singh(SHUBHAMSINGH10) `

Output :

```Not a BST
```

Thanks to for suggesting above solution.

METHOD 4(Using In-Order Traversal)
Thanks to LJW489 for suggesting this method.
1) Do In-Order Traversal of the given tree and store the result in a temp array.
3) Check if the temp array is sorted in ascending order, if it is, then the tree is BST.

Time Complexity: O(n)

We can avoid the use of Auxiliary Array. While doing In-Order traversal, we can keep track of previously visited node. If the value of the currently visited node is less than the previous value, then tree is not BST. Thanks to ygos for this space optimization.

## C

 `bool` `isBST(``struct` `node* root) ` `{ ` `    ``static` `struct` `node *prev = NULL; ` `     `  `    ``// traverse the tree in inorder fashion and keep track of prev node ` `    ``if` `(root) ` `    ``{ ` `        ``if` `(!isBST(root->left)) ` `          ``return` `false``; ` ` `  `        ``// Allows only distinct valued nodes  ` `        ``if` `(prev != NULL && root->data <= prev->data) ` `          ``return` `false``; ` ` `  `        ``prev = root; ` ` `  `        ``return` `isBST(root->right); ` `    ``} ` ` `  `    ``return` `true``; ` `} `

## Java

 `// Java implementation to check if given Binary tree ` `// is a BST or not ` ` `  `/* Class containing left and right child of current ` ` ``node and key value*/` `class` `Node ` `{ ` `    ``int` `data; ` `    ``Node left, right; ` ` `  `    ``public` `Node(``int` `item) ` `    ``{ ` `        ``data = item; ` `        ``left = right = ``null``; ` `    ``} ` `} ` ` `  `public` `class` `BinaryTree ` `{ ` `    ``// Root of the Binary Tree ` `    ``Node root; ` ` `  `    ``// To keep tract of previous node in Inorder Traversal ` `    ``Node prev; ` ` `  `    ``boolean` `isBST()  { ` `        ``prev = ``null``; ` `        ``return` `isBST(root); ` `    ``} ` ` `  `    ``/* Returns true if given search tree is binary ` `       ``search tree (efficient version) */` `    ``boolean` `isBST(Node node) ` `    ``{ ` `        ``// traverse the tree in inorder fashion and ` `        ``// keep a track of previous node ` `        ``if` `(node != ``null``) ` `        ``{ ` `            ``if` `(!isBST(node.left)) ` `                ``return` `false``; ` ` `  `            ``// allows only distinct values node ` `            ``if` `(prev != ``null` `&& node.data <= prev.data ) ` `                ``return` `false``; ` `            ``prev = node; ` `            ``return` `isBST(node.right); ` `        ``} ` `        ``return` `true``; ` `    ``} ` ` `  `    ``/* Driver program to test above functions */` `    ``public` `static` `void` `main(String args[]) ` `    ``{ ` `        ``BinaryTree tree = ``new` `BinaryTree(); ` `        ``tree.root = ``new` `Node(``4``); ` `        ``tree.root.left = ``new` `Node(``2``); ` `        ``tree.root.right = ``new` `Node(``5``); ` `        ``tree.root.left.left = ``new` `Node(``1``); ` `        ``tree.root.left.right = ``new` `Node(``3``); ` ` `  `        ``if` `(tree.isBST()) ` `            ``System.out.println(``"IS BST"``); ` `        ``else` `            ``System.out.println(``"Not a BST"``); ` `    ``} ` `} `

## Python3

 `# Python implementation to check if  ` `# given Binary tree is a BST or not ` ` `  `# A binary tree node containing data ` `# field, left and right pointers ` `class` `Node: ` `    ``# constructor to create new node ` `    ``def` `__init__(``self``, val): ` `        ``self``.data ``=` `val ` `        ``self``.left ``=` `None` `        ``self``.right ``=` `None` ` `  `# global variable prev - to keep track ` `# of previous node during Inorder  ` `# traversal ` `prev ``=` `None` ` `  `# function to check if given binary ` `# tree is BST ` `def` `isbst(root): ` `     `  `    ``# prev is a global variable ` `    ``global` `prev ` `    ``prev ``=` `None` `    ``return` `isbst_rec(root) ` ` `  ` `  `# Helper function to test if binary ` `# tree is BST ` `# Traverse the tree in inorder fashion  ` `# and keep track of previous node ` `# return true if tree is Binary  ` `# search tree otherwise false ` `def` `isbst_rec(root): ` `     `  `    ``# prev is a global variable ` `    ``global` `prev  ` ` `  `    ``# if tree is empty return true ` `    ``if` `root ``is` `None``: ` `        ``return` `True` ` `  `    ``if` `isbst_rec(root.left) ``is` `False``: ` `        ``return` `False` ` `  `    ``# if previous node'data is found  ` `    ``# greater than the current node's ` `    ``# data return fals ` `    ``if` `prev ``is` `not` `None` `and` `prev.data > root.data: ` `        ``return` `False` ` `  `    ``# store the current node in prev ` `    ``prev ``=` `root ` `    ``return` `isbst_rec(root.right) ` ` `  ` `  `# driver code to test above function ` `root ``=` `Node(``4``) ` `root.left ``=` `Node(``2``) ` `root.right ``=` `Node(``5``) ` `root.left.left ``=` `Node(``1``) ` `root.left.right ``=` `Node(``3``) ` ` `  `if` `isbst(root): ` `    ``print``(``"is BST"``) ` `else``: ` `    ``print``(``"not a BST"``) ` ` `  `# This code is contributed by ` `# Shweta Singh(shweta44) `

The use of static variable can also be avoided by using reference to prev node as a parameter.

 `// C++ program to check if a given tree is BST. ` `#include ` `using` `namespace` `std; ` ` `  `/* A binary tree node has data, pointer to ` `left child and a pointer to right child */` `struct` `Node ` `{ ` `    ``int` `data; ` `    ``struct` `Node* left, *right; ` `     `  `    ``Node(``int` `data) ` `    ``{ ` `        ``this``->data = data; ` `        ``left = right = NULL; ` `    ``} ` `}; ` ` `  ` `  `bool` `isBSTUtil(``struct` `Node* root, Node *&prev) ` `{ ` `    ``// traverse the tree in inorder fashion and  ` `    ``// keep track of prev node ` `    ``if` `(root) ` `    ``{ ` `        ``if` `(!isBSTUtil(root->left, prev)) ` `          ``return` `false``; ` `  `  `        ``// Allows only distinct valued nodes  ` `        ``if` `(prev != NULL && root->data <= prev->data) ` `          ``return` `false``; ` `  `  `        ``prev = root; ` `  `  `        ``return` `isBSTUtil(root->right, prev); ` `    ``} ` `  `  `    ``return` `true``; ` `} ` ` `  `bool` `isBST(Node *root) ` `{ ` `   ``Node *prev = NULL; ` `   ``return` `isBSTUtil(root, prev); ` `} ` ` `  `/* Driver program to test above functions*/` `int` `main() ` `{ ` `    ``struct` `Node *root = ``new` `Node(3); ` `    ``root->left     = ``new` `Node(2); ` `    ``root->right     = ``new` `Node(5); ` `    ``root->left->left = ``new` `Node(1); ` `    ``root->left->right = ``new` `Node(4); ` ` `  `    ``if` `(isBST(root)) ` `        ``cout << ``"Is BST"``; ` `    ``else` `        ``cout << ``"Not a BST"``; ` ` `  `    ``return` `0; ` `} `

Please write comments if you find any bug in the above programs/algorithms or other ways to solve the same problem.