diff --git a/DIRECTORY.md b/DIRECTORY.md
index bf6e738e..0436e929 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -703,6 +703,8 @@
     * [Test Is Unique](https://github.com/BrianLusina/PythonSnips/blob/master/pystrings/is_unique/test_is_unique.py)
   * Issubsequence
     * [Test Is Subsequence](https://github.com/BrianLusina/PythonSnips/blob/master/pystrings/issubsequence/test_is_subsequence.py)
+  * Longest Self Contained Substring
+    * [Test Longest Self Contained Substring](https://github.com/BrianLusina/PythonSnips/blob/master/pystrings/longest_self_contained_substring/test_longest_self_contained_substring.py)
   * Look And Say Sequence
     * [Test Look And Say Sequence](https://github.com/BrianLusina/PythonSnips/blob/master/pystrings/look_and_say_sequence/test_look_and_say_sequence.py)
   * Max Vowels In Substring
diff --git a/pystrings/length_of_longest_substring/README.md b/pystrings/length_of_longest_substring/README.md
index 85e1b8a9..084dd35f 100644
--- a/pystrings/length_of_longest_substring/README.md
+++ b/pystrings/length_of_longest_substring/README.md
@@ -1,4 +1,4 @@
-Longest Substring Without Repeating Characters
+# Longest Substring Without Repeating Characters
 
 Given a string s, find the length of the longest substring without repeating characters.
 
diff --git a/pystrings/longest_self_contained_substring/README.md b/pystrings/longest_self_contained_substring/README.md
new file mode 100644
index 00000000..b8e6fd15
--- /dev/null
+++ b/pystrings/longest_self_contained_substring/README.md
@@ -0,0 +1,112 @@
+# Find Longest Self-Contained Substring
+
+You are given a string, s, consisting of lowercase English letters. Your task is to find the length of the longest 
+self-contained substring of s.
+
+A substring t of s is called self-contained if:
+- t is not equal to the entire string s.
+- Every character in t does not appear anywhere else in s (outside of t).
+
+In other words, all characters in t are completely unique to that substring within the string s.
+Return the length of the longest self-contained substring. If no such substring exists, return -1.
+
+Constraints:
+
+- 2 ≤ s.length ≤ 1000
+- s consists only of lowercase English letters.
+
+## Examples
+
+![Example 1](./images/longest_self_contained_substring_example_1.png)
+![Example 2](./images/longest_self_contained_substring_example_2.png)
+![Example 3](./images/longest_self_contained_substring_example_3.png)
+![Example 4](./images/longest_self_contained_substring_example_4.png)
+![Example 5](./images/longest_self_contained_substring_example_5.png)
+![Example 6](./images/longest_self_contained_substring_example_6.png)
+![Example 7](./images/longest_self_contained_substring_example_7.png)
+![Example 8](./images/longest_self_contained_substring_example_8.png)
+
+---
+
+## Solution
+
+We iterate through the string once to record where each character first appears and where it last appears. Two separate 
+hash maps store each character’s first and last occurrence indices. Each unique character serves as a potential starting 
+point, defining an initial window from its first to last occurrence. The window is adjusted based on the following 
+conditions:
+
+- As we iterate within this window, if we encounter a character whose last occurrence is further to the right than the 
+current window’s end, we expand the window to include it. This ensures that the substring remains self-contained, 
+meaning all occurrences of each character within it are included. The process continues until no more characters 
+extend the window’s boundary. 
+
+- As we expand the window, every character inside it must have its first occurrence within the current window’s start 
+boundary. If we encounter a character whose first occurrence index is before the current window’s starting position, 
+it means that an earlier part of the string contains an instance of that character, violating the self-contained 
+property. When this happens, the current substring is invalid, and we discard it from consideration. 
+
+The maximum valid window length is tracked and updated accordingly, ensuring the longest valid substring is returned.
+
+The steps of the algorithm are as follows:
+
+1. Create two hashmaps, first, and last, to store the first and last occurrence index of each character in the string. 
+2. Iterate through the string once to populate these hash maps with each character’s first and last occurrence. 
+3. Initialize max_len = -1 to keep track of the maximum length of a valid self-contained substring found. 
+4. For each unique character c1 (processed once), start from its first occurrence index:
+
+   - Initialize the start and end of the window by setting the starting point to the character’s first occurrence and the 
+   ending point to its last occurrence in the string.
+
+   - Iterate through the string from the starting position start, extending the endpoint end whenever a character’s last 
+   occurrence is beyond the current endpoint end.
+
+   - If a character c2 inside the window has its first occurrence before the window's start, the window is invalid.
+
+5. Validate the substring:
+
+   - When the current index j reaches the end, check if the window is valid and its length is less than the total string 
+   length.
+
+   - If the window is valid, update max_len with the maximum of its current value and the window’s length (end - start + 1).
+
+6. After checking all potential starting characters, return the maximum valid length found. If no valid substring exists, 
+return -1.
+
+Let’s look at the illustration below to better understand the solution.
+
+![Solution 1](./images/solution/longest_self_contained_substring_solution_1.png)
+![Solution 2](./images/solution/longest_self_contained_substring_solution_2.png)
+![Solution 3](./images/solution/longest_self_contained_substring_solution_3.png)
+![Solution 4](./images/solution/longest_self_contained_substring_solution_4.png)
+![Solution 5](./images/solution/longest_self_contained_substring_solution_5.png)
+![Solution 6](./images/solution/longest_self_contained_substring_solution_6.png)
+![Solution 7](./images/solution/longest_self_contained_substring_solution_7.png)
+![Solution 8](./images/solution/longest_self_contained_substring_solution_8.png)
+![Solution 9](./images/solution/longest_self_contained_substring_solution_9.png)
+![Solution 10](./images/solution/longest_self_contained_substring_solution_10.png)
+![Solution 11](./images/solution/longest_self_contained_substring_solution_11.png)
+![Solution 12](./images/solution/longest_self_contained_substring_solution_12.png)
+![Solution 13](./images/solution/longest_self_contained_substring_solution_13.png)
+![Solution 14](./images/solution/longest_self_contained_substring_solution_14.png)
+![Solution 15](./images/solution/longest_self_contained_substring_solution_15.png)
+![Solution 16](./images/solution/longest_self_contained_substring_solution_16.png)
+![Solution 17](./images/solution/longest_self_contained_substring_solution_17.png)
+![Solution 18](./images/solution/longest_self_contained_substring_solution_18.png)
+![Solution 19](./images/solution/longest_self_contained_substring_solution_19.png)
+![Solution 20](./images/solution/longest_self_contained_substring_solution_20.png)
+![Solution 21](./images/solution/longest_self_contained_substring_solution_21.png)
+![Solution 22](./images/solution/longest_self_contained_substring_solution_22.png)
+![Solution 23](./images/solution/longest_self_contained_substring_solution_23.png)
+![Solution 24](./images/solution/longest_self_contained_substring_solution_24.png)
+![Solution 25](./images/solution/longest_self_contained_substring_solution_25.png)
+![Solution 26](./images/solution/longest_self_contained_substring_solution_26.png)
+![Solution 27](./images/solution/longest_self_contained_substring_solution_27.png)
+![Solution 28](./images/solution/longest_self_contained_substring_solution_28.png)
+
+### Time Complexity
+
+The time complexity of the above solution is O(n), where n is the number of characters in the string.
+
+### Space Complexity
+
+The space complexity of the above solution is O(1) because of the fixed character set size, 26 lowercase English letters.
\ No newline at end of file
diff --git a/pystrings/longest_self_contained_substring/__init__.py b/pystrings/longest_self_contained_substring/__init__.py
new file mode 100644
index 00000000..6a9a9b8f
--- /dev/null
+++ b/pystrings/longest_self_contained_substring/__init__.py
@@ -0,0 +1,117 @@
+def longest_self_contained_substring(s: str) -> int:
+    """
+    Finds the longest self-contained substring in a given string.
+
+    A self-contained substring is one where each character only appears within the substring itself.
+    The function returns the length of the longest self-contained substring, and -1 if no such substring exists.
+
+    Parameters:
+        s (str): The input string.
+
+    Returns:
+        int: The length of the longest self-contained substring, or -1 if no such substring exists.
+
+    Examples:
+        >>> longest_self_contained_substring("xyyx")
+        2
+        >>> longest_self_contained_substring("xyxy")
+        -1
+        >>> longest_self_contained_substring("abacd")
+        4
+
+    Note:
+        This implementation uses a brute-force approach with O(n³) time complexity.
+        For better performance, consider using max_substring_length() which runs in O(n).
+    """
+    n = len(s)
+
+    # First, find the first and last occurrence of each character
+    # This helps us quickly check if a character appears outside a range
+    first_occurrence = {}
+    last_occurrence = {}
+
+    for i, char in enumerate(s):
+        if char not in first_occurrence:
+            first_occurrence[char] = i
+        last_occurrence[char] = i
+
+    max_length = -1
+
+    # Try all possible substrings (excluding the entire string)
+    for start in range(n):
+        for end in range(start, n):
+            # Skip the entire string
+            if start == 0 and end == n - 1:
+                continue
+
+            # Check if this substring is self-contained
+            substring = s[start:end + 1]
+            is_self_contained = True
+
+            # For each character in the substring, verify it doesn't appear outside
+            for char in set(substring):
+                # If the character's first occurrence is before our start
+                # or last occurrence is after our end, it appears outside
+                if first_occurrence[char] < start or last_occurrence[char] > end:
+                    is_self_contained = False
+                    break
+
+            # If self-contained, update our maximum
+            if is_self_contained:
+                max_length = max(max_length, end - start + 1)
+
+    return max_length
+
+
+def max_substring_length(s: str) -> int:
+    """
+    Finds the length of the longest substring of s that is self-contained.
+
+    A self-contained substring is one in which all characters only appear within the substring.
+
+    The function uses an optimized window expansion approach. For each unique character as a starting point,
+    it defines an initial window from the character's first to last occurrence. The window is expanded to include
+    all occurrences of characters within it, and is invalidated if any character's first occurrence lies before
+    the window start.
+
+    Parameters:
+        s (str): The string to find the longest self-contained substring of
+
+    Returns:
+        int: The length of the longest self-contained substring of s
+
+    Examples:
+        >>> max_substring_length("xyyx")
+        2
+        >>> max_substring_length("xyxy")
+        -1
+        >>> max_substring_length("abacd")
+        4
+
+    Note:
+        Time complexity: O(n), Space complexity: O(1) for fixed character set size.
+    """
+    first = {}
+    last = {}
+    for i, c in enumerate(s):
+        if c not in first:
+            first[c] = i
+        last[c] = i
+
+    max_len = -1
+
+    for c1 in first:
+        start = first[c1]
+        end = last[c1]
+        j = start
+
+        while j < len(s):
+            c2 = s[j]
+            if first[c2] < start:
+                break
+            end = max(end, last[c2])
+            if end == j and end - start + 1 != len(s):
+                max_len = max(max_len, end - start + 1)
+            j += 1
+
+    return max_len
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diff --git a/pystrings/longest_self_contained_substring/test_longest_self_contained_substring.py b/pystrings/longest_self_contained_substring/test_longest_self_contained_substring.py
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+++ b/pystrings/longest_self_contained_substring/test_longest_self_contained_substring.py
@@ -0,0 +1,142 @@
+import unittest
+from . import longest_self_contained_substring, max_substring_length
+
+
+class LongestSelfContainedSubstringTestCase(unittest.TestCase):
+    def test_1(self):
+        s = "xyyx"
+        expected = 2
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_2(self):
+        s = "xyxy"
+        expected = -1
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_3(self):
+        s = "abacd"
+        expected = 4
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_4(self):
+        s = "aabbcc"
+        expected = 4
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_5(self):
+        s = "xyzxy"
+        expected = 1
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_6(self):
+        s = "abcde"
+        expected = 4
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_7(self):
+        s = "aaaa"
+        expected = -1
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_8(self):
+        s = "aabccbdd"
+        expected = 6
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_9(self):
+        s = "abcdefghigklmnopqrstuvwxyz"
+        expected = 25
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_10(self):
+        s = "abcabcabc"
+        expected = -1
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+    def test_11(self):
+        s = "aaabbbcccddd"
+        expected = 9
+        actual = longest_self_contained_substring(s)
+        self.assertEqual(expected, actual)
+
+
+class MaxSelfContainedSubstringTestCase(unittest.TestCase):
+    def test_1(self):
+        s = "xyyx"
+        expected = 2
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_2(self):
+        s = "xyxy"
+        expected = -1
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_3(self):
+        s = "abacd"
+        expected = 4
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_4(self):
+        s = "aabbcc"
+        expected = 4
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_5(self):
+        s = "xyzxy"
+        expected = 1
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_6(self):
+        s = "abcde"
+        expected = 4
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_7(self):
+        s = "aaaa"
+        expected = -1
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_8(self):
+        s = "aabccbdd"
+        expected = 6
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_9(self):
+        s = "abcdefghigklmnopqrstuvwxyz"
+        expected = 25
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_10(self):
+        s = "abcabcabc"
+        expected = -1
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+    def test_11(self):
+        s = "aaabbbcccddd"
+        expected = 9
+        actual = max_substring_length(s)
+        self.assertEqual(expected, actual)
+
+
+if __name__ == '__main__':
+    unittest.main()