diff --git a/src/Solution.java b/src/Solution.java
index 046213d..3cc3e06 100644
--- a/src/Solution.java
+++ b/src/Solution.java
@@ -1,2 +1,15 @@
+import java.util.HashSet;
+import java.util.Set;
+
 public class Solution {
+    public int lengthOfLongestSubstring(String s) {
+        if (s != null){
+            Set<Character> stringList = new HashSet<>();
+            for (int i = 0; i < s.length(); i++) {
+                stringList.add(s.charAt(i));
+            }
+            return stringList.size();
+        }
+        return 0;
+    }
 }