diff --git a/src/leetcode/design.md b/src/leetcode/design.md
index f936956..ecd659f 100644
--- a/src/leetcode/design.md
+++ b/src/leetcode/design.md
@@ -1 +1,42 @@
-# Design
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+# Design
+
++ [Flatten Nested List Iterator](#flatten-nested-list-iterator)
+
+## Flatten Nested List Iterator
+https://leetcode.com/problems/flatten-nested-list-iterator/
+```java
+public class NestedIterator implements Iterator<Integer> {
+
+    private final List<NestedInteger> nestedList;
+    private NestedIterator iterator;
+    private int index;
+    public NestedIterator(List<NestedInteger> nestedList) {
+        this.nestedList = nestedList;
+        this.iterator = null;
+        this.index = 0;
+    }
+
+    @Override
+    public Integer next() {
+        if(nestedList.get(index).isInteger()) return nestedList.get(index++).getInteger();
+        return iterator.next();
+    }
+
+    @Override
+    public boolean hasNext() {
+        if (nestedList.size() > index && nestedList.get(index) != null) {
+            if (nestedList.get(index).isInteger()) return true;
+
+            if (iterator == null) iterator = new NestedIterator(nestedList.get(index).getList());
+
+            if (iterator.hasNext()) return true;
+
+            index++;
+            iterator = null;
+            return this.hasNext();
+        }
+        return false;
+    }
+}
+
+```
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