docs(docs): add longest‑substring problem page and sliding‑window reference with SVG illustrations

apps/docs/content/(hash-table)/3-longest-substring-without-repeating-characters.mdx

@@ -0,0 +1,47 @@
+---
+title: '3. Longest Substring Without Repeating Characters'
+description: Given a string s, find the length of the longest substring without duplicate characters
+sidebar:
+  label: 'Longest Substring Without Repeating Characters'
+  badge: 'Medium'
+---
+
+<Badge variant="accent">Sliding Window</Badge>
+
+### 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.
+
+## Solution
+
+```py
+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
+```