Clarify the problem constraints (e.g., case sensitivity, duplicates, empty strings) and then propose an efficient solution using a hash set for the allowed characters. Iterate through each string, checking if all its characters are in the set, and collect the valid strings. Discuss time and space complexity, and consider edge cases.
Pro tip: Mention that converting the allowed characters to a hash set gives O(1) lookups, and that you can early-exit the inner loop as soon as an invalid character is found. Also, discuss whether to preserve the original order of strings and how to handle duplicates.
Ask about case sensitivity, whether the set can contain duplicates, if the output should preserve order, and how to handle empty strings or empty set.
Use a hash set for the allowed characters to enable O(1) membership checks. The result can be a list or array, depending on the required output format.
For each string, iterate through its characters and check if each is in the allowed set. If all characters are valid, add the string to the result. Use early termination for efficiency.
Calculate time complexity as O(N * L) where N is number of strings and L is average length, and space complexity as O(M) for the set and O(K) for the result. Discuss edge cases like empty input, strings with invalid characters, and Unicode.
Walk through examples, including edge cases. Consider optimizations like bitmask if the character set is small (e.g., lowercase letters) to reduce space and improve speed.
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