Create 98_Validate-BST.cpp (#356)

C++/98_Validate-BST.cpp

@@ -0,0 +1,36 @@
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+class Solution {
+public:
+    bool isValidBST(TreeNode* root, TreeNode* minNode = nullptr, TreeNode* maxNode = nullptr) {
+        // Base case: an empty tree is a valid BST
+        if (root == nullptr) return true;
+
+        // If there is a restriction on the minimum value
+        if (minNode != nullptr && root->val <= minNode->val) return false;
+        // If there is a restriction on the maximum value
+        if (maxNode != nullptr && root->val >= maxNode->val) return false;
+
+        // Recursively validate the left and right subtrees
+        return isValidBST(root->left, minNode, root) && isValidBST(root->right, root, maxNode);
+    }
+};
+/*
+    2
+   / \
+  1   3
+  Expected : True
+
+    10
+   / \
+  5   15
+     / \
+    6   20
+  Expected : False
+
+*/