I started writing a brute-force combination generator before I even clocked the 0-8 range thing.
Use backtracking to replace each '?' with digits 0-8, pruning branches where the partial sum plus the minimum possible sum of remaining digits exceeds the target or plus the maximum possible sum is less than the target. Collect all valid strings where the total sum equals the target. This approach efficiently explores only feasible combinations.
Pro tip: Clarify upfront that '?' can be replaced by digits 0-8 (not 0-9) and that the target sum applies to all digits including fixed ones. Also discuss handling edge cases like no solution or empty string.
Confirm the range for '?' (0-8), that fixed digits are included in the sum, and ask about output format (list of strings, order, duplicates).
Calculate the sum of fixed digits and count the number of '?' to determine the remaining sum needed and the number of digits to assign.
Recursively assign digits 0-8 to each '?', tracking the current sum. Prune if current sum + min possible remaining > target or current sum + max possible remaining < target.
Write the recursive function, ensuring base case when all '?' are replaced and sum equals target. Test with examples and edge cases.
Discuss time complexity (worst-case 9^k where k is number of '?') and how pruning reduces it. Mention potential optimizations like memoization if needed.
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