String-based

For a “string-based” interview mention, do not fabricate a particular LeetCode problem. Clarify the task: validate a format, locate a pattern, compare two inputs, transform text, or optimize a substring. Then identify the right invariant. Character counts solve permutation-style questions; two pointers solve symmetric comparisons; a sliding window solves contiguous constraints; a stack helps nested delimiters; parser state helps tokenization. Make the treatment of whitespace, punctuation, case, empty strings, and Unicode explicit because these often change correctness. Prefer a single linear pass when possible and avoid creating many substrings in a loop if the language copies them. A direct scan is normally O(n) time; a frequency map adds O(u) space for u distinct characters. This response is intentionally a framework because the stored question has no concrete specification; an honest clarification is better preparation than a confident invented solution.