diff --git a/Algorithms/slidingWindow/longestUnique.py b/Algorithms/slidingWindow/longestUnique.py
new file mode 100644
index 0000000..5b6abec
--- /dev/null
+++ b/Algorithms/slidingWindow/longestUnique.py
@@ -0,0 +1,35 @@
+from collections import OrderedDict
+from pydantic import BaseModel, Field
+from typing import List
+
+class SlidingWindowParam(BaseModel):
+    s: str = Field(..., description="Input string", min_length=1)
+
+
+def lengthOfLongestSubstringTwoDistinct(s: SlidingWindowParam) -> int:
+    seen = OrderedDict()
+    start = 0
+    longest = 0
+    for end in range(len(s)):
+        if s[end] in seen:
+            seen.move_to_end(s[end])
+        seen[s[end]] = end
+        if len(seen) > 2:
+            _, idx = seen.popitem(last=False)
+            start = idx+1
+        longest = max(longest, end-start+1)
+    return longest
+
+def maximumUniqueSubarray(nums: List[int]) -> int:
+    last_seen = dict()
+    start, curr_sum, max_sum = 0, 0, 0
+    for end in range(len(nums)):
+        if nums[end] in last_seen:
+            last_idx = last_seen[nums[end]] 
+            if last_idx >= start:
+                curr_sum -= sum(nums[start:(last_idx+1)])
+                start = last_idx + 1
+        curr_sum += nums[end]
+        max_sum = max(max_sum, curr_sum)
+        last_seen[nums[end]] = end
+    return max_sum
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diff --git a/tests/test_longest_substring.py b/tests/test_longest_substring.py
new file mode 100644
index 0000000..d59b649
--- /dev/null
+++ b/tests/test_longest_substring.py
@@ -0,0 +1,164 @@
+import pytest
+from pydantic import ValidationError
+from Algorithms.slidingWindow.longestUnique import lengthOfLongestSubstringTwoDistinct, maximumUniqueSubarray, SlidingWindowParam
+
+# -------------------------
+# Basic correctness tests
+# -------------------------
+
+@pytest.mark.parametrize(
+    "input_str, expected",
+    [
+        ("eceba", 3),
+        ("ccaabbb", 5),
+        ("ababffzzeee", 5),  
+        ("aa", 2),
+        ("a", 1),
+        ("ab", 2),
+        ("abc", 2),
+    ],
+)
+def test_basic_cases(input_str, expected):
+    param = SlidingWindowParam(s=input_str)
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == expected
+
+@pytest.mark.parametrize(
+    "nums, expected",
+    [
+        ([4, 2, 4, 5, 6], 17),          # example 1
+        ([5, 2, 1, 2, 5, 2, 1, 2, 5], 8),  # example 2
+        ([1], 1),
+        ([1, 2], 3),
+        ([1, 1], 1),
+        ([1, 2, 3], 6),
+        ([3, 3, 3], 3),
+    ],
+)
+def test_basic_cases(nums, expected):
+    assert maximumUniqueSubarray(nums) == expected
+
+# -------------------------
+# Regression / tricky cases
+# -------------------------
+
+def test_alternating_characters():
+    param = SlidingWindowParam(s="abababab")
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == 8
+
+
+def test_three_distinct_repeated():
+    param = SlidingWindowParam(s="abcabcabc")
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == 2
+
+
+def test_eviction_order_matters():
+    # Breaks implementations that don't move_to_end
+    param = SlidingWindowParam(s="abaccc")
+    # Longest valid substring: "accc"
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == 4
+
+
+def test_multiple_evictions():
+    param = SlidingWindowParam(s="aabbccddeeff")
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == 4
+
+def test_duplicate_at_start():
+    # Forces window to jump start
+    nums = [2, 1, 2, 3, 4]
+    # Best: [1,2,3,4]
+    assert maximumUniqueSubarray(nums) == 10
+
+
+def test_duplicate_at_end():
+    nums = [1, 2, 3, 4, 1]
+    # Best: [1,2,3,4]
+    assert maximumUniqueSubarray(nums) == 10
+
+
+def test_multiple_duplicates():
+    nums = [1, 2, 1, 3, 2, 3, 4]
+    # Best: [1,3,2,3,4] is invalid
+    # Best valid: [1,3,2] or [2,3,4]
+    assert maximumUniqueSubarray(nums) == 9
+
+
+def test_eviction_jump_needed():
+    # Breaks incorrect start updates
+    nums = [1, 2, 3, 2, 5]
+    # Best: [3,2,5]
+    assert maximumUniqueSubarray(nums) == 10
+
+# -------------------------
+# Edge cases
+# -------------------------
+
+def test_all_same_character():
+    param = SlidingWindowParam(s="aaaaaaa")
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == 7
+
+
+def test_two_characters_only():
+    param = SlidingWindowParam(s="abababababab")
+    assert lengthOfLongestSubstringTwoDistinct(param.s) == 12
+
+def test_all_unique():
+    nums = [1, 2, 3, 4, 5, 6]
+    assert maximumUniqueSubarray(nums) == sum(nums)
+
+
+def test_all_same():
+    nums = [7, 7, 7, 7]
+    assert maximumUniqueSubarray(nums) == 7
+
+
+def test_large_values():
+    nums = [10_000, 9_999, 10_000]
+    assert maximumUniqueSubarray(nums) == 19_999
+
+def test_strictly_increasing_then_repeat():
+    nums = list(range(1, 100)) + [50]
+    assert maximumUniqueSubarray(nums) == sum(range(1, 100))
+
+
+def test_alternating_pattern():
+    nums = [1, 2] * 50
+    assert maximumUniqueSubarray(nums) == 3  # [1,2]
+
+# -------------------------
+# Pydantic validation tests
+# -------------------------
+
+def test_empty_string_rejected():
+    with pytest.raises(ValidationError):
+        SlidingWindowParam(s="")
+
+
+def test_non_string_input():
+    with pytest.raises(ValidationError):
+        SlidingWindowParam(s=123)
+
+
+# -------------------------
+# Invariant-style tests
+# -------------------------
+
+@pytest.mark.parametrize("s", [
+    "abc",
+    "aabcbcdbca",
+    "xyzzzyx",
+    "pwwkew",
+])
+def test_result_never_exceeds_string_length(s):
+    param = SlidingWindowParam(s=s)
+    result = lengthOfLongestSubstringTwoDistinct(param.s)
+    assert 0 < result <= len(s)
+
+@pytest.mark.parametrize("nums", [
+    [1, 2, 3],
+    [1, 2, 1, 2, 3],
+    [5, 5, 5, 1, 2],
+    [100, 200, 300, 100, 400],
+])
+def test_result_never_exceeds_sum(nums):
+    result = maximumUniqueSubarray(nums)
+    assert 0 < result <= sum(nums)
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