diff --git a/JungHwan/Day21/NeetCode_LongestSubstringWithoutDuplicates.py b/JungHwan/Day21/NeetCode_LongestSubstringWithoutDuplicates.py
new file mode 100644
index 0000000..15de090
--- /dev/null
+++ b/JungHwan/Day21/NeetCode_LongestSubstringWithoutDuplicates.py
@@ -0,0 +1,13 @@
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        l, r = 0, 0
+        maxl = 0
+        while r < len(s):
+            for i in range(l, r):
+                if s[r] == s[i]: #found duplicate
+                    l = r = i + 1
+                    break
+                #duplicate not found
+            maxl = max(maxl, r - l + 1)
+            r += 1
+        return maxl
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