diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index 6dec650..ee4c0f9 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -152,4 +152,5 @@ mod p222_count_complete_tree_nodes;
 mod p199_binary_tree_right_side_view;
 mod p637_average_of_levels_in_binary_tree;
 mod p530_minimum_absolute_difference_in_bst;
-mod p230_kth_smallest_element_in_a_bst;
\ No newline at end of file
+mod p230_kth_smallest_element_in_a_bst;
+mod p98_validate_binary_search_tree;
\ No newline at end of file
diff --git a/src/problem/p98_validate_binary_search_tree.rs b/src/problem/p98_validate_binary_search_tree.rs
new file mode 100644
index 0000000..e86b398
--- /dev/null
+++ b/src/problem/p98_validate_binary_search_tree.rs
@@ -0,0 +1,67 @@
+/**
+ * [98] Validate Binary Search Tree
+ */
+pub struct Solution {}
+
+use crate::util::tree::{TreeNode, to_tree};
+
+// submission codes start here
+
+// Definition for a binary tree node.
+// #[derive(Debug, PartialEq, Eq)]
+// pub struct TreeNode {
+//   pub val: i32,
+//   pub left: Option<Rc<RefCell<TreeNode>>>,
+//   pub right: Option<Rc<RefCell<TreeNode>>>,
+// }
+//
+// impl TreeNode {
+//   #[inline]
+//   pub fn new(val: i32) -> Self {
+//     TreeNode {
+//       val,
+//       left: None,
+//       right: None
+//     }
+//   }
+// }
+use std::rc::Rc;
+use std::cell::RefCell;
+impl Solution {
+    fn inorder_iterate(node: &Rc<RefCell<TreeNode>>, array: &mut Vec<i32>) {
+        if let Some(left) = node.borrow().left.as_ref() {
+            Self::inorder_iterate(left, array);
+        }   
+        array.push(node.borrow().val);
+        if let Some(right) = node.borrow().right.as_ref() {
+            Self::inorder_iterate(right, array);
+        }
+    }
+
+    pub fn is_valid_bst(root: Option<Rc<RefCell<TreeNode>>>) -> bool {
+        let mut array = vec![];
+
+        if let Some(root) = root {
+            Self::inorder_iterate(&root, &mut array);
+        }
+
+        for i in 1..array.len() {
+            if array[i - 1] >= array[i] {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_98() {
+    }
+}