diff --git a/hw01/sp/Trie.java b/hw01/sp/Trie.java
new file mode 100644
index 0000000..4ebfd31
--- /dev/null
+++ b/hw01/sp/Trie.java
@@ -0,0 +1,32 @@
+package sp;
+ 
+public interface Trie {
+ 
+    /**
+     * Expected complexity: O(|element|)
+     * @return <tt>true</tt> if this set did not already contain the specified
+     *         element
+     */
+    boolean add(String element);
+ 
+    /**
+     * Expected complexity: O(|element|)
+     */
+    boolean contains(String element);
+ 
+    /**
+     * Expected complexity: O(|element|)
+     * @return <tt>true</tt> if this set contained the specified element
+     */
+    boolean remove(String element);
+ 
+    /**
+     * Expected complexity: O(1)
+     */
+    int size();
+ 
+    /**
+     * Expected complexity: O(|prefix|)
+     */
+    int howManyStartsWithPrefix(String prefix);
+}
\ No newline at end of file
diff --git a/hw01/sp/TrieImpl.java b/hw01/sp/TrieImpl.java
new file mode 100644
index 0000000..975b358
--- /dev/null
+++ b/hw01/sp/TrieImpl.java
@@ -0,0 +1,96 @@
+package sp;
+
+import java.lang.Character;
+import java.lang.ref.WeakReference;
+import java.util.HashMap;
+import java.util.TreeSet;
+import java.util.Map;
+ 
+public class TrieImpl implements Trie {
+
+    Node root = new Node();
+ 
+    public boolean add(String element) {
+        Node node = root;
+        for(char c : element.toCharArray()) 
+            node = node.next(c);
+        return node.setLeaf(true);
+    }
+ 
+    public boolean contains(String element) {
+        Node node = root;
+        for(char c : element.toCharArray()) {
+            if (!node.containsKey(c))
+                return false;
+            node = node.get(c);
+        }
+        return node.isLeaf();
+    }
+ 
+    public boolean remove(String element) {
+        Node node = root;
+        for(char c : element.toCharArray()) {
+            if (!node.containsKey(c))
+                return false;
+            node = node.get(c);
+        }
+        return node.setLeaf(false);
+    }
+ 
+    public int size() {
+        return root.getSize();
+    }
+ 
+    public int howManyStartsWithPrefix(String prefix) {
+        Node node = root;
+        for(char c : prefix.toCharArray()) {
+            if (!node.containsKey(c))
+                return 0;
+            node = node.get(c);
+        }
+        return node.getSize();
+    }
+
+    private class Node extends HashMap<Character, Node> {
+
+        private int wordCount = 0;
+        private boolean isLeaf = false;
+        private WeakReference<Node> parrent = null;
+        private Character character = null;
+
+        public Node() {}
+
+        public Node(Node prev, Character c) { 
+            parrent = new WeakReference<>(prev); 
+            character = c;
+        }
+
+        public Node next(Character key) {
+            if (!containsKey(key)) 
+                put(key, new Node(this, key));
+            return get(key);
+        }
+
+        public int getSize() { return wordCount; }
+
+        private void updateSize(Character c, int delta) {
+            if (c != null && get(c).getSize() == 0) {
+                remove(c);
+            }
+            wordCount += delta;
+            if (parrent != null)
+                parrent.get().updateSize(this.character, delta);
+        }
+
+        boolean isLeaf() { return isLeaf; }
+
+        boolean setLeaf(boolean value) {
+            if (isLeaf == value)
+                return false;
+            isLeaf = value;          
+            updateSize(null, value ? 1 : -1);  
+            return true;
+        }
+
+    }
+}
\ No newline at end of file
diff --git a/hw01/sp/TrieTest.java b/hw01/sp/TrieTest.java
new file mode 100644
index 0000000..d0012dd
--- /dev/null
+++ b/hw01/sp/TrieTest.java
@@ -0,0 +1,197 @@
+package sp;
+ 
+import static org.junit.Assert.*;
+ 
+public class TrieTest {
+ 
+    @org.junit.Test
+    public void testAdd() throws Exception {
+        Trie trie = new TrieImpl();
+
+        assertTrue(trie.add(""));
+        assertTrue(trie.add("aa"));
+        assertTrue(trie.add("a"));
+        assertTrue(trie.add("aaa"));
+        assertTrue(trie.add("aab"));
+        assertTrue(trie.add("aac"));
+        assertTrue(trie.add("ab"));
+        
+        assertFalse(trie.add(""));
+        assertFalse(trie.add("aa"));
+        assertFalse(trie.add("a"));
+        assertFalse(trie.add("aaa"));
+        assertFalse(trie.add("aab"));
+        assertFalse(trie.add("aac"));
+        assertFalse(trie.add("ab"));
+
+        trie.remove("");
+        trie.remove("aab");
+
+        assertTrue(trie.add("aaaa"));
+        assertFalse(trie.add("aaaa"));
+
+        trie.remove("a");
+        trie.remove("aa");
+        trie.remove("aaa");
+        trie.remove("aac");
+        trie.remove("ab");
+    }
+
+    @org.junit.Test
+    public void testRemove() throws Exception {
+        Trie trie = new TrieImpl();
+        assertFalse(trie.contains(""));
+        trie.add("");
+        trie.add("aa");
+        trie.add("a");
+        trie.add("aaa");
+        trie.add("aab");
+        trie.add("aac");
+        trie.add("ab");
+        assertTrue(trie.remove(""));
+        assertTrue(trie.remove("aab"));
+        trie.add("aaaa");
+        assertTrue(trie.remove("a"));
+        assertTrue(trie.remove("aa"));
+        assertTrue(trie.remove("aaa"));
+        assertTrue(trie.remove("aac"));
+        assertTrue(trie.remove("ab"));
+
+        assertFalse(trie.remove(""));
+        assertFalse(trie.remove("aab"));
+        assertFalse(trie.remove(""));
+        assertFalse(trie.remove("aab"));
+        assertFalse(trie.remove("a"));
+        assertFalse(trie.remove("aa"));
+        assertFalse(trie.remove("aaa"));
+        assertFalse(trie.remove("aac"));
+        assertFalse(trie.remove("ab"));
+    }    
+
+    @org.junit.Test
+    public void testContains() throws Exception {
+        Trie trie = new TrieImpl();
+        assertFalse(trie.contains(""));
+        trie.add("");
+        trie.add("aa");
+        trie.add("a");
+        trie.add("aaa");
+        trie.add("aab");
+        trie.add("aac");
+        trie.add("ab");
+
+        assertTrue(trie.contains(""));
+        assertTrue(trie.contains("aa"));
+        assertTrue(trie.contains("a"));
+        assertTrue(trie.contains("aaa"));
+        assertTrue(trie.contains("aab"));
+        assertTrue(trie.contains("aac"));
+        assertTrue(trie.contains("ab"));
+
+        trie.remove("");
+        trie.remove("aab");
+
+        assertFalse(trie.contains(""));
+        assertTrue(trie.contains("aa"));
+        assertTrue(trie.contains("a"));
+        assertTrue(trie.contains("aaa"));
+        assertFalse(trie.contains("aab"));
+        assertTrue(trie.contains("aac"));
+        assertTrue(trie.contains("ab"));
+
+        trie.add("aaaa");
+        trie.remove("a");
+        trie.remove("aa");
+        trie.remove("aaa");
+        trie.remove("aac");
+        trie.remove("ab");
+
+        assertFalse(trie.contains(""));
+        assertFalse(trie.contains("aa"));
+        assertFalse(trie.contains("a"));
+        assertFalse(trie.contains("aaa"));
+        assertFalse(trie.contains("aab"));
+        assertFalse(trie.contains("aac"));
+        assertFalse(trie.contains("ab"));
+        assertTrue(trie.contains("aaaa"));
+    }
+
+    @org.junit.Test
+    public void testSize() throws Exception {
+        Trie trie = new TrieImpl();
+        
+        assertEquals(trie.size(), 0);
+        trie.add("");
+        assertEquals(trie.size(), 1);
+        trie.add("aa");
+        trie.add("a");
+        trie.add("aaa");
+        assertEquals(trie.size(), 4);
+        trie.add("aab");
+        trie.add("aac");
+        trie.add("ab");
+        assertEquals(trie.size(), 7);
+        
+        trie.remove("");
+        trie.remove("aab");
+        assertEquals(trie.size(), 5);
+
+        trie.add("aaaa");
+        assertEquals(trie.size(), 6);
+
+        trie.remove("a");
+        trie.remove("aa");
+        trie.remove("aaa");
+        assertEquals(trie.size(), 3);
+        trie.remove("aac");
+        trie.remove("ab");
+        assertEquals(trie.size(), 1);
+
+    }
+ 
+    @org.junit.Test
+    public void testHowManyStartsWithPrefix() throws Exception {
+        Trie trie = new TrieImpl();
+        
+        assertEquals(trie.howManyStartsWithPrefix(""), 0);
+        assertEquals(trie.howManyStartsWithPrefix("a"), 0);
+        assertEquals(trie.howManyStartsWithPrefix("aa"), 0);
+        assertEquals(trie.howManyStartsWithPrefix("aaa"), 0);
+
+        trie.add("");
+        trie.add("aa");
+        trie.add("a");
+        trie.add("aaa");
+        trie.add("aab");
+        trie.add("aac");
+        trie.add("ab");
+
+        assertEquals(trie.howManyStartsWithPrefix(""), 7);
+        assertEquals(trie.howManyStartsWithPrefix("a"), 6);
+        assertEquals(trie.howManyStartsWithPrefix("aa"), 4);
+        assertEquals(trie.howManyStartsWithPrefix("aaa"), 1);
+        
+        trie.remove("");
+        trie.remove("aab");
+
+        trie.add("aaaa");
+
+        assertEquals(trie.howManyStartsWithPrefix(""), 6);
+        assertEquals(trie.howManyStartsWithPrefix("a"), 6);
+        assertEquals(trie.howManyStartsWithPrefix("aa"), 4);
+        assertEquals(trie.howManyStartsWithPrefix("aaa"), 2);
+
+        trie.remove("a");
+        trie.remove("aa");
+        trie.remove("aaa");
+        trie.remove("aac");
+        trie.remove("ab");
+
+        assertEquals(trie.howManyStartsWithPrefix(""), 1);
+        assertEquals(trie.howManyStartsWithPrefix("a"), 1);
+        assertEquals(trie.howManyStartsWithPrefix("aa"), 1);
+        assertEquals(trie.howManyStartsWithPrefix("aaa"), 1);
+    }
+
+
+}
\ No newline at end of file