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Often the insight is not the algorithm but that sorting first makes an intractable problem a single pass - three-sum, merge intervals, and longest string chain all begin with a sort. Interviewers care that you can name the resulting O(n log n) and justify why the extra pass is free. Custom comparators, and knowing when a counting sort beats a comparison sort, are the senior-level follow-ups.
Order matters, duplicates need grouping, or a custom comparator unlocks a one-pass scan.
15 problems. Each one runs in an in-browser editor against real test cases, with hints and a worked solution.
The patterns cheat sheet covers all of these in one page - what each one looks like in a problem statement, and the template that follows.