diff --git a/239_sliding_window_maximum.py b/239_sliding_window_maximum.py
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+++ b/239_sliding_window_maximum.py
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+"""
+LeetCode Problem #239 — Sliding Window Maximum (Hard)
+-----------------------------------------------------
+You are given an array nums and an integer k.
+Return the maximum value in each sliding window of size k.
+
+Example:
+Input: nums = [1,3,-1,-3,5,3,6,7], k = 3
+Output: [3,3,5,5,6,7]
+
+"""
+
+from collections import deque
+
+class Solution:
+    def maxSlidingWindow(self, nums, k):
+        n = len(nums)
+        if not nums or k == 0:
+            return []
+
+        dq = deque()  # stores indices
+        result = []
+
+        for i in range(n):
+            # Remove elements outside the window (left side)
+            while dq and dq[0] < i - k + 1:
+                dq.popleft()
+
+            # Remove all elements smaller than current (not useful)
+            while dq and nums[dq[-1]] < nums[i]:
+                dq.pop()
+
+            # Add current element's index
+            dq.append(i)
+
+            # Append current window's max to result
+            if i >= k - 1:
+                result.append(nums[dq[0]])
+
+        return result
+
+
+# Example usage
+if __name__ == "__main__":
+    nums = [1,3,-1,-3,5,3,6,7]
+    k = 3
+    print("Sliding Window Maximum:", Solution().maxSlidingWindow(nums, k))