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String problems are usually another pattern in disguise - a sliding window, a hash map of counts, or two pointers - with extra parsing care. The reliable habits are reaching for a frequency map on anything anagram-shaped and being precise about what counts as a character. Know the mutability and cost model of strings in your chosen language, because interviewers do ask.
String input where the work is scanning, comparing, or counting characters.
49 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.