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+"""
+3. Longest Substring Without Repeating Characters
+Difficulty: Medium
+https://leetcode.com/problems/longest-substring-without-repeating-characters/
+
+──────────────────────────────────────────────────
+
+Given a string s, find the length of the longest substring without
+duplicate characters.
+
+
+
+Example 1:
+
+Input: s = "abcabcbb"
+Output: 3
+Explanation: The answer is "abc", with the length of 3. Note that
+"bca" and "cab" are also correct answers.
+
+Example 2:
+
+Input: s = "bbbbb"
+Output: 1
+Explanation: The answer is "b", with the length of 1.
+
+Example 3:
+
+Input: s = "pwwkew"
+Output: 3
+Explanation: The answer is "wke", with the length of 3.
+Notice that the answer must be a substring, "pwke" is a subsequence
+and not a substring.
+
+
+
+Constraints:
+
+        • 0 <= s.length <= 10^5
+
+        • s consists of English letters, digits, symbols and spaces.
+"""
+
+
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        left = 0
+        ans = 0
+        seen = set()
+        for right, c in enumerate(s):
+            while c in seen:
+                seen.remove(s[left])
+                left += 1
+            seen.add(c)
+            ans = max(ans, right - left + 1)
+
+        return ans