Sliding Window

This is a premium interview-preparation course built around one insight: when a problem asks about the best contiguous subarray or substring, you rarely need to re-examine a range from scratch. A **sliding window** carries just enough state — a running sum, a frequency map, a monotonic deque — and updates it in O(1) as the window moves, collapsing an O(n^2) brute force into O(n). You will start with the mental model (fixed vs variable windows, when to expand and when to shrink, and the two-pointer relationship), then work through curated problems grouped by the patterns interviewers actually test: fixed windows, variable windows, frequency-driven windows, the exactly-K trick, and premium deque and prefix-hybrid problems. Every lesson identifies the pattern first, visualises the window, then shows a clean Java 17 implementation.