Sliding Window

You are asked for the highest-scoring stretch of five games in a row, or the longest run of distinct characters, or the smallest slice of an array that adds up to at least some number. All three are about a contiguous run, a window, and the brute force is to check every possible window from scratch. That re-does an enormous amount of work, because neighboring windows overlap almost entirely.

The sliding window technique keeps a running summary of the current window and slides it one step at a time: add what enters on the right, remove what leaves on the left. Nothing gets rescanned, so a problem that looked quadratic collapses to a single linear pass. It is really the same-direction two-pointer idea, aimed specifically at contiguous subarrays and substrings.

There are two shapes. A fixed window has a set size and just slides along. A variable window grows and shrinks to find the best run under some condition. The hard part is rarely the code; it is recognizing that a problem is a window at all, and knowing when to grow versus when to shrink. That is what this unit builds.