Added a new c++ file for check is it BST or not

Bst.cpp

@@ -0,0 +1,78 @@
+//code link:-https://www.geeksforgeeks.org/a-program-to-check-if-a-binary-tree-is-bst-or-not/
+
+//A program to check if a binary tree is BST or not
+
+#include <bits/stdc++.h>
+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);
+
+	// Function call
+	if (isBST(root))
+		cout << "Is BST";
+	else
+		cout << "Not a BST";
+
+	return 0;
+}
+
+// This code is contributed by rathbhupendra