diff --git a/src/leetcode/design.md b/src/leetcode/design.md
index f936956..700fb47 100644
--- a/src/leetcode/design.md
+++ b/src/leetcode/design.md
@@ -1 +1,44 @@
-# Design
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+# Design
+
++ [Binary Search Tree Iterator](#binary-search-tree-iterator)
+
+## Binary Search Tree Iterator
+https://leetcode.com/problems/binary-search-tree-iterator/
+```java
+class BSTIterator {
+
+    TreeNode root;
+    int index;
+    List<TreeNode> flatTree = new ArrayList<>();
+
+    public BSTIterator(TreeNode root) {
+        index = 0;
+        this.root = root;
+        flattening(root);
+    }
+
+    public int next() {
+
+        TreeNode node = flatTree.get(index);
+        index++;
+        return node.val;
+    }
+
+    public boolean hasNext() {
+        return index < flatTree.size();
+    }
+
+    private void flattening(TreeNode root) {
+        if (root == null) return;
+        if (root.left != null) {
+            flattening(root.left);
+        }
+
+        flatTree.add(root);
+
+        if (root.right != null) {
+            flattening(root.right);
+        }
+    }
+}
+```
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