diff --git a/0003-longest-substring-without-repeating-characters/0003-longest-substring-without-repeating-characters.py b/0003-longest-substring-without-repeating-characters/0003-longest-substring-without-repeating-characters.py
new file mode 100644
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+++ b/0003-longest-substring-without-repeating-characters/0003-longest-substring-without-repeating-characters.py
@@ -0,0 +1,40 @@
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+
+        """
+        Complexities:
+        Time:  O(n)
+        Space: O(X)
+
+        where n = len(s)
+              X = len([All compatible characters])
+        """
+
+        charsInSubstring = set({})
+
+        startIndex = 0  # inclusive
+        endIndex = 0    # exclusive
+        maxLengthFound = 0
+
+        # Use sliding window to find solution
+        # - O(n) -> startIndex or endIndex always increase by 1 every iteration = 2n
+        while endIndex < len(s):
+            # if next letter not in substring, then endIndex++
+            if s[endIndex] not in charsInSubstring:
+                charsInSubstring.add(s[endIndex])
+                endIndex += 1
+
+                # Update maxLengthFound
+                if endIndex - startIndex > maxLengthFound:
+                    maxLengthFound = endIndex - startIndex
+            
+            # else pop off front until popped [next letter]
+            else:
+                while s[startIndex] != s[endIndex]:
+                    charsInSubstring.remove(s[startIndex])
+                    startIndex += 1
+
+                charsInSubstring.remove(s[startIndex])
+                startIndex += 1
+
+        return maxLengthFound
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