update week 2 starter code

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-starter/src/iterators/concat_iterator.rs

@@ -0,0 +1,49 @@
+#![allow(unused_variables)] // TODO(you): remove this lint after implementing this mod
+#![allow(dead_code)] // TODO(you): remove this lint after implementing this mod
+
+use std::sync::Arc;
+
+use anyhow::Result;
+
+use super::StorageIterator;
+use crate::table::{SsTable, SsTableIterator};
+
+/// Concat multiple iterators ordered in key order and their key ranges do not overlap. We do not want to create the
+/// iterators when initializing this iterator to reduce the overhead of seeking.
+pub struct SstConcatIterator {
+    current: Option<SsTableIterator>,
+    next_sst_idx: usize,
+    sstables: Vec<Arc<SsTable>>,
+}
+
+impl SstConcatIterator {
+    pub fn create_and_seek_to_first(sstables: Vec<Arc<SsTable>>) -> Result<Self> {
+        unimplemented!()
+    }
+
+    pub fn create_and_seek_to_key(sstables: Vec<Arc<SsTable>>, key: &[u8]) -> Result<Self> {
+        unimplemented!()
+    }
+}
+
+impl StorageIterator for SstConcatIterator {
+    fn key(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn value(&self) -> &[u8] {
+        unimplemented!()
+    }
+
+    fn is_valid(&self) -> bool {
+        unimplemented!()
+    }
+
+    fn next(&mut self) -> Result<()> {
+        unimplemented!()
+    }
+
+    fn num_active_iterators(&self) -> usize {
+        1
+    }
+}