How do you recognize when to use the sliding window pattern?

Look for these signals in the problem statement: the input is a linear data structure like an array or string, you need to find a contiguous subarray or substring, the problem asks for "longest," "shortest," "maximum," or "minimum" of something satisfying a condition, or there is a constraint on what the window can contain like "at most k distinct characters" or "sum at least target." If the problem asks about subsequences rather than subarrays, sliding window does not apply because subsequences are not contiguous. ---