diff --git a/include/s0098_validate_binary_search_tree.hpp b/include/s0098_validate_binary_search_tree.hpp
new file mode 100644
index 0000000..aa5356f
--- /dev/null
+++ b/include/s0098_validate_binary_search_tree.hpp
@@ -0,0 +1,11 @@
+#ifndef S0098_VALIDATE_BINARY_SEARCH_TREE_HPP
+#define S0098_VALIDATE_BINARY_SEARCH_TREE_HPP
+
+#include "structures.hpp"
+
+class S0098 {
+ public:
+  bool isValidBST(TreeNode* root);
+};
+
+#endif
diff --git a/src/s0098_validate_binary_search_tree.cpp b/src/s0098_validate_binary_search_tree.cpp
new file mode 100644
index 0000000..6efaf43
--- /dev/null
+++ b/src/s0098_validate_binary_search_tree.cpp
@@ -0,0 +1,35 @@
+#include "s0098_validate_binary_search_tree.hpp"
+
+struct TreeNodeState_S0098 {
+  int maxVal;
+  int minVal;
+  bool isValidBST;
+};
+
+TreeNodeState_S0098 isValidBSTHelper(TreeNode* root) {
+  if (root == nullptr) return TreeNodeState_S0098{0, 0, false};
+  if (root->left == nullptr && root->right != nullptr) {
+    TreeNodeState_S0098 rightState = isValidBSTHelper(root->right);
+    return TreeNodeState_S0098{
+        rightState.maxVal, root->val,
+        root->val < rightState.minVal && rightState.isValidBST};
+  } else if (root->left != nullptr && root->right == nullptr) {
+    TreeNodeState_S0098 leftState = isValidBSTHelper(root->left);
+    return TreeNodeState_S0098{
+        root->val, leftState.minVal,
+        root->val > leftState.maxVal && leftState.isValidBST};
+  } else if (root->left != nullptr && root->right != nullptr) {
+    TreeNodeState_S0098 leftState = isValidBSTHelper(root->left);
+    TreeNodeState_S0098 rightState = isValidBSTHelper(root->right);
+    return TreeNodeState_S0098{
+        rightState.maxVal, leftState.minVal,
+        root->val > leftState.maxVal && root->val < rightState.minVal &&
+            leftState.isValidBST && rightState.isValidBST};
+  } else {
+    return TreeNodeState_S0098{root->val, root->val, true};
+  }
+}
+
+bool S0098::isValidBST(TreeNode* root) {
+  return isValidBSTHelper(root).isValidBST;
+}