Add BST Checker problem

Author: sahilbansal17

BST Checker/README.md

@@ -0,0 +1,34 @@
+# Binary Search Tree Checker
+[Try the problem](https://leetcode.com/problems/validate-binary-search-tree/)
+
+Given a binary tree, determine if it is a valid binary search tree (BST).
+
+Assume a BST is defined as follows:
+- 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.
+
+### Example 1
+
+```
+    2
+   / \
+  1   3
+
+Input: [2,1,3]
+Output: true
+```
+
+### Example 2
+
+```
+    5
+   / \
+  1   4
+     / \
+    3   6
+
+Input: [5,1,4,null,null,3,6]
+Output: false
+Explanation: The root node's value is 5 but its right child's value is 4.
+```

BST Checker/solution.cpp

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+#include <iostream>
+using namespace std;
+
+/**
+ * Definition for a binary tree node.
+ */
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+TreeNode* insertLeft(TreeNode* root, int value) {
+    root->left = new TreeNode(value);
+    return root->left;
+}
+
+TreeNode* insertRight(TreeNode* root, int value) {
+    root->right = new TreeNode(value);
+    return root->right;
+}
+
+class Solution {
+public:
+    bool isValid(TreeNode* root, int minVal, int maxVal, bool noMin, bool noMax) {
+        if (root == NULL) {
+            return true;
+        }
+        if (!noMin && root->val <= minVal) {
+            return false;
+        }
+        if (!noMax && root->val >= maxVal) {
+            return false;
+        }
+        bool rightValid = false;
+        bool leftValid = false;
+        rightValid = isValid(root->right, root->val, maxVal, false, noMax);
+        leftValid = isValid(root->left, minVal, root->val, noMin, false);
+        return rightValid && leftValid;
+    }
+    bool isValidBST(TreeNode* root) {
+        return isValid(root, 0, 0, true, true);
+    }
+};
+
+int main() {
+    auto root1 = new TreeNode(50);
+    Solution solver;
+    cout << solver.isValidBST(root1) << endl; // true
+
+    auto root2 = new TreeNode(50);
+    insertLeft(root2, 30);
+    insertLeft(root2->left, 10);
+    insertRight(root2, 70);
+    insertRight(root2->right, 80);
+    insertRight(root2->left, 40);
+    insertLeft(root2->right, 60);
+    cout << solver.isValidBST(root2) << endl; // true
+
+    auto root3 = new TreeNode(50);
+    insertLeft(root3, 30);
+    insertLeft(root3->left, 20);
+    insertRight(root3, 80);
+    insertRight(root3->right, 90);
+    insertRight(root3->left, 60);
+    insertLeft(root3->right, 70);
+    cout << solver.isValidBST(root3) << endl; // false
+}

README.md

@@ -6,4 +6,5 @@ This repository contains the coding interview problems along with solutions.
 | [Island Count](Island%20Count) | | [CPP](Island%20Count/solution.cpp) | Graph Traversal | Medium |
 | [Minimum Falling Path Sum](Minimum%20Falling%20Path%20Sum) | [text](Minimum%20Falling%20Path%20Sum/solution.txt) | [CPP](Minimum%20Falling%20Path%20Sum/solution.cpp) | Dynamic Programming | Medium |
 | [Word Break](Word%20Break) | [text](Word%20Break/solution.txt) | [CPP](Word%20Break/solution.cpp) | Dynamic Programming | Medium |
-| [Remove K Digits](Remove%20K%20Digits) | [text](Remove%20K%20Digits/solution.txt) | [CPP](Remove%20K%20Digits/solution.cpp) | Greedy, Stack | Medium |
+| [Remove K Digits](Remove%20K%20Digits) | [text](Remove%20K%20Digits/solution.txt) | [CPP](Remove%20K%20Digits/solution.cpp) | Greedy, Stack | Medium |
+| [Binary Search Tree Checker](BST%20Checker) | To be updated | [CPP](BST%20Checker/solution.cpp) | Trees | Medium |