added validate binary search tree algorithm

algorithms/CPlusPlus/README.md

@@ -152,6 +152,7 @@
 - [Iterative Segment Tree](Trees/IterativeSegmentTree.cpp)
 - [Print all nodes at level k](Trees/print-all-nodes-at-a-level.cpp)
 - [Sum of right leaves](Trees/sum-of-right-leaves.cpp)
+- [Validate Binary Search Tree](Trees/validate-binary-search-tree.cpp)
 
 ## Trie
 

algorithms/CPlusPlus/Trees/validate-binary-search-tree.cpp

@@ -0,0 +1,60 @@
+#include <iostream>
+using namespace std;
+
+// sturcture of a tree node
+struct Node
+{
+    int data;
+    Node *right;
+    Node *left;
+
+    Node(int data)
+    {
+        this->data = data;
+        this->right = NULL;
+        this->left = NULL;
+    }
+};
+
+// function to check if a tree is a BST or not
+bool validateBST(Node *root)
+{
+    // if the tree is empty
+    if (root == NULL)
+        return true;
+
+    // if the left child is not null and its data is greater than the root's data then its not a BST
+    if (root->left != NULL && root->left->data > root->data)
+        return false;
+
+    // if the right child is not null and its data is less than the root's data then its not a BST
+    if (root->right != NULL && root->right->data < root->data)
+        return false;
+
+    // if the left and right subtree are not BST then the tree is not a BST
+    if (!validateBST(root->left) || !validateBST(root->right))
+        return false;
+
+    // if all the above conditions are false then the tree is a BST
+    return true;
+}
+
+int main()
+{
+    // creating a bst
+    Node *root = new Node(5);
+    root->left = new Node(3);
+    root->right = new Node(7);
+    root->left->left = new Node(2);
+    root->left->right = new Node(4);
+    root->right->left = new Node(6);
+    root->right->right = new Node(8);
+
+    // checking if the tree is a BST or not
+    if (validateBST(root))
+        cout << "Valid BST" << endl;
+    else
+        cout << "Invalid BST" << endl;
+
+    return 0;
+}