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Given a string s, you may transform each letter to lowercase or uppercase (digits stay unchanged). Return a list of all strings that can be created this way.
Results follow the order in which choices are explored (lowercase before uppercase at each letter).
A medium backtracking problem, graded against 6 test cases (4 of them hidden).
Systematic exploration of all candidates, abandoning partial ones that cannot work.
Reach for it when you see: "All permutations/combinations/subsets", or constraint puzzles like N-Queens and Sudoku.
More Backtracking problems →Binary backtracking over letters. Walk the string; for a digit, recurse without branching. For a letter, set it lowercase and recurse, then uppercase and recurse. Record the string when you pass the end.
Time: O(2ᴸ·n) where L is the letter count. Space: O(n).
The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.
These apply to the pattern as a whole, not just this problem.