At-Most-K Window: When to Use It

Start with the idea

A problem is a good match for At-Most-K Window when it asks for longest range with bounded distinct items. Here the concrete task is: Return the longest subarray containing at most two distinct values. The visible input [1,2,1,2,3,2,2] is an example, not a definition of the whole problem. Ask what operations the solution must support and what the input guarantees. Choose this family when a valid window can be repaired by moving one boundary; non-monotone constraints may need a different method. The right choice follows from those guarantees, not from a familiar noun in the prompt.

The decisive invariant is: Keep a frequency state and shrink while more than k distinct values are present. Try to say why it applies to the task without quoting the implementation. Which fact becomes known after one update, and which possible answers remain? A window is a contiguous interval with a maintained summary. Describe what entering the right edge adds and what leaving the left edge removes. If that claim cannot be maintained on a legal input, the pattern is wrong or one of its preconditions is missing. A counterexample is more useful than a vague warning: change one property of the sample, trace the first two steps, and identify the exact claim that fails.

Compare costs before committing. The advertised bound is O(n) expected time · O(k) space; identify the operation responsible for it and the extra memory it needs.