diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index 8051d6d..bc02c4c 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -146,4 +146,5 @@ mod p101_symmetric_tree;
 mod p114_flatten_binary_tree_to_linked_list;
 mod p112_path_sum;
 mod p129_sum_root_to_leaf_numbers;
-mod p124_binary_tree_maximum_path_sum;
\ No newline at end of file
+mod p124_binary_tree_maximum_path_sum;
+mod p173_binary_search_tree_iterator;
\ No newline at end of file
diff --git a/src/problem/p173_binary_search_tree_iterator.rs b/src/problem/p173_binary_search_tree_iterator.rs
new file mode 100644
index 0000000..a4428db
--- /dev/null
+++ b/src/problem/p173_binary_search_tree_iterator.rs
@@ -0,0 +1,117 @@
+/**
+ * [173] Binary Search Tree Iterator
+ */
+pub struct Solution {}
+
+use tokio::time::error::Elapsed;
+
+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;
+
+struct BSTIterator {
+    stack: Vec<Rc<RefCell<TreeNode>>>
+}
+
+/**
+ * `&self` means the method takes an immutable reference.
+ * If you need a mutable reference, change it to `&mut self` instead.
+ */
+impl BSTIterator {
+
+    fn new(root: Option<Rc<RefCell<TreeNode>>>) -> Self {
+        if root.is_none() {
+            return BSTIterator {
+                stack: vec![]
+            };
+        }   
+
+        let mut stack = vec![];
+        let root = root.unwrap();
+        let mut node = root;
+
+        loop {
+            stack.push(Rc::clone(&node));
+
+            let left = if let Some(left) = &node.borrow().left {
+                Rc::clone(left)
+            } else {
+                break;
+            };
+
+            node = left;
+        }
+
+
+        BSTIterator {
+            stack
+        }
+    }
+    
+    fn next(&mut self) -> i32 {
+        let node = self.stack.pop().unwrap();
+        let val = node.borrow().val;
+        
+        if let Some(right) = &node.borrow().right {
+            let mut node = Rc::clone(right);
+
+            loop {
+                self.stack.push(Rc::clone(&node));
+
+                let left = if let Some(left) = &node.borrow().left {
+                    Rc::clone(left)
+                } else {
+                    break;
+                };
+
+                node = left;
+            }
+        }
+
+        val
+    }
+    
+    fn has_next(&self) -> bool {
+        !self.stack.is_empty()
+    }
+}
+
+/**
+ * Your BSTIterator object will be instantiated and called as such:
+ * let obj = BSTIterator::new(root);
+ * let ret_1: i32 = obj.next();
+ * let ret_2: bool = obj.has_next();
+ */
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_173() {
+    }
+}