Problem 174: Validate if a binary tree is binary search tree

Author: mandliya

README.md

@@ -6,7 +6,7 @@
 
 | Current Status|     Stats     |
 | :------------: | :----------: |
-| Total Problems | 173 |
+| Total Problems | 174 |
 
 </center>
 
@@ -129,6 +129,7 @@ Include contains single header implementation of data structures and some algori
 | Invert a binary tree, recursively and iteratively.| [invert_a_tree.cpp](tree_problems/invert_a_tree.cpp) |
 | Given a Binary Search Tree, find ceil and floor of a given key in it. If the given key lie in the BST, then both floor and ceil is equal to that key, else ceil is equal to next greater key (if any) in the BST and floor is equal to previous greater key (if any) in the BST | [floor_ceil_bst.cpp](tree_problems/floor_ceil_bst.cpp) |
 | Find kth smallest element in a binary search tree | [kth_smallest.cpp](tree_problems/kth_smallest.cpp)|
+| Validate if a given binary tree is a binary search tree. | [validate_bst.cpp](tree_problems/validate_bst.cpp) |
 
 ### String Problems
 | Problem | Solution |

tree_problems/validate_bst.cpp

@@ -0,0 +1,95 @@
+/*
+ * Validate if a given binary tree is a binary search tree.
+ * 
+ * Example:
+ * 
+ *          10
+ *       /     \
+ *     5       15
+ * 
+ * Answer: True
+ * 
+ * Example:
+ * 
+ *          10
+ *       /     \
+ *     5        15
+ *   /   \     /   \
+ *  2     7   9     17
+ * 
+ * Answer : False                    
+ */
+
+#include <iostream>
+
+struct TreeNode {
+    int data;
+    TreeNode* left;
+    TreeNode* right;
+    TreeNode(int d) :
+        data{d}, left{nullptr}, right{nullptr} {}
+};
+
+bool is_valid_bst(TreeNode* root, TreeNode* min, TreeNode* max)
+{
+    if (root == nullptr) {
+        return true;
+    }
+
+    if (min != nullptr && root->data <= min->data) {
+        return false;
+    }
+
+    if (max != nullptr && root->data >= max->data) {
+        return false;
+    }
+
+    return is_valid_bst(root->left, min, root) && is_valid_bst(root->right, root, max);
+
+}
+
+bool is_valid_bst(TreeNode* root)
+{
+    return is_valid_bst(root, nullptr, nullptr);
+}
+
+void print_inorder(TreeNode* root)
+{
+    if (root != nullptr) {
+        print_inorder(root->left);
+        std::cout << root->data << " ";
+        print_inorder(root->right);
+    }
+}
+
+int main()
+{
+    TreeNode* root1 = new TreeNode(10);
+    root1->left = new TreeNode(5);
+    root1->right = new TreeNode(15);
+    std::cout << "Inorder traversal of tree: ";
+    print_inorder(root1);
+    std::cout << std::endl;
+    if (is_valid_bst(root1)) {
+        std::cout << "Above tree is a valid binary search tree.\n";  
+    } else {
+        std::cout << "Above tree is not a valid binary search tree.\n";
+    }
+
+    TreeNode* root2 = new TreeNode(10);
+    root2->left = new TreeNode(5);
+    root2->right = new TreeNode(15);
+    root2->left->left = new TreeNode(2);
+    root2->left->right = new TreeNode(7);
+    root2->right->left = new TreeNode(9);
+    root2->right->right = new TreeNode(17);
+     std::cout << "Inorder traversal of tree: ";
+    print_inorder(root2);
+    std::cout << std::endl;
+    if (is_valid_bst(root2)) {
+        std::cout << "Above tree is a valid binary search tree.\n";  
+    } else {
+        std::cout << "Above tree is not a valid binary search tree.\n";
+    }
+    return 0;
+}