I knew this was a subset enumeration problem pretty quickly but fumbled explaining why bitmasks work here.
Start by clarifying the problem and constraints, then propose a backtracking solution that explores all subsequences while pruning branches with duplicate characters. Optimize using bitmasking to represent character sets and memoization to avoid redundant work.
Pro tip: Mention that you can preprocess strings to remove any with duplicate characters, and use a bitmask to quickly check for overlaps. This shows attention to optimization and practical coding skills.
Ask clarifying questions about input size, character set, and whether strings can be skipped. Validate that the goal is to maximize total length with all unique characters.
Filter out strings that have duplicate characters internally, as they can never be part of a valid concatenation. Convert each remaining string to a bitmask of its characters.
Use recursion to explore including or excluding each string. Maintain a current bitmask of used characters and only include a string if its bitmask doesn't overlap with the current one.
Use memoization to cache results for a given index and current bitmask, avoiding redundant computations. This reduces time complexity significantly.
Discuss time and space complexity: O(2^N * N) without memoization, but with memoization it's O(2^26 * N) in worst case, which is manageable. Space is O(2^26) for memo table.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.