diff --git a/csharp/0003-longest-substring-without-repeating-characters.cs b/csharp/0003-longest-substring-without-repeating-characters.cs
index cf6e82d7d..15f410758 100644
--- a/csharp/0003-longest-substring-without-repeating-characters.cs
+++ b/csharp/0003-longest-substring-without-repeating-characters.cs
@@ -1,14 +1,22 @@
-public class Solution {
-    public int LengthOfLongestSubstring(string s) {
+// hashset
+public class Solution
+{
+    public int LengthOfLongestSubstring(string s)
+    {
         int leftPointer = 0, rightPointer = 0, maxLength = 0;
         HashSet<int> chars = new HashSet<int>();
 
-        while (rightPointer < s.Length) {
+        while (rightPointer < s.Length)
+        {
             char currChar = s[rightPointer];
-            if (chars.Contains(currChar)) { // Move left pointer until all duplicate chars removed
+            if (chars.Contains(currChar))
+            { 
+                // Move left pointer until all duplicate chars removed
                 chars.Remove(s[leftPointer]);
                 leftPointer++;
-            } else {
+            }
+            else
+            {
                 chars.Add(currChar);
                 maxLength = Math.Max(maxLength, rightPointer - leftPointer + 1);
                 rightPointer++;
@@ -16,4 +24,38 @@ public int LengthOfLongestSubstring(string s) {
         }
         return maxLength;
     }
+}
+
+//bitmask
+public class Solution
+{
+    private Int128 ONE = 1;
+    public int LengthOfLongestSubstring(string s)
+    {
+        int Convert(char ch) => ch - ' ';
+        Int128 mask = 0;
+        int l = 0, r = 0, output = 0;
+        while (r < s.Length)
+        {
+            Int128 temp = mask ^ (ONE << Convert(s[r]));
+            if (temp < mask)
+            {
+                while (s[l] != s[r])
+                {
+                    mask ^= ONE << Convert(s[l]);
+                    l++;
+                }
+                mask ^= ONE << Convert(s[l]);
+                l++;
+            }
+            else
+            {
+                mask = temp;
+                output = Math.Max(output, r - l + 1);
+                r++;
+            }
+        }
+
+        return output;
+    }
 }
\ No newline at end of file