Started with the obvious O(n^2) loop comparing every pair character by character, which they accepted but clearly wanted more.
Start by clarifying the problem constraints (e.g., input size, character set) and then propose an efficient algorithm. A common approach is to represent each string as a bitmask of its characters, then count pairs whose bitwise AND is zero. Discuss time and space complexity and possible optimizations.
Pro tip: Mention that since strings are lowercase, a 26-bit integer suffices, enabling fast bitwise operations. Also, consider precomputing masks and using a frequency map to handle duplicates efficiently.
Ask about input size, whether strings can be empty, and if duplicates are allowed. This determines the optimal approach.
Represent each string as a bitmask where each bit indicates presence of a character. This reduces the problem to counting pairs with disjoint masks.
Use a frequency map of masks. For each mask, iterate over all possible masks (or use subset enumeration) to find disjoint ones, or use a hash map to count pairs efficiently.
Discuss time and space complexity. Compare brute-force O(n^2) with bitmask approach O(n * 2^26) or O(n * 2^26) is too large; instead, use a map and iterate over subsets of complement.
Consider empty strings, strings with all unique characters, and large inputs. Walk through a small example to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.