LEETCODE 2022 Added solution for binary-search-tree-iterator

src/leetcode/design.md

@@ -1 +1,44 @@
-# Design
+# 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);
+        }
+    }
+}
+```