Add sliding-window module: 12 sliding window and two-pointer interview problems, brute-force cross-checks, dry-run traces, operation counts, pitfalls (11 tests)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b6EqSkUWKNBc8i3Q3U

sliding-window/output/09-pitfalls.txt

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+Four classic mistakes, each reproduced
+======================================
+
+1. longest substring without repeats, left = lastSeen + 1 WITHOUT Math.max
+   input "abba":  buggy = 3   correct = 2
+   at the final 'a', lastSeen('a') = 0, so the buggy code moves left from 2 back to 1
+
+2. minimum-size subarray window used on input that contains negatives
+   target 3, input [-2, -2, 3]:  window = 0 ("none")   brute force = 1 ([3])
+   the running sum is -1 when the 3 arrives, so the window never looks valid
+
+3. 3Sum without skipping duplicates
+   input [-1, 0, 1, 2, -1, -4]
+   buggy   = [[-1, -1, 2], [-1, 0, 1], [-1, 0, 1]]
+   correct = [[-1, -1, 2], [-1, 0, 1]]
+
+4. two pointers on an array that is NOT sorted
+   input [3, 2, 4], target 6:  twoSumSorted = []   but 2 + 4 = 6 exists (brute force: true)