Can you generate all binary strings from a given pattern?
Candidates must write code to produce all possible binary strings that match a specific pattern containing wildcards or fixed bits. This tests recursion and backtracking skills.
Why Interviewers Ask This
This question evaluates a candidate's grasp of recursive backtracking and state management. Interviewers look for the ability to explore the entire solution space systematically without missing combinations. It also tests how well a candidate handles string manipulation and manages the call stack depth in recursive implementations.
How to Answer This Question
Explain the recursive function structure where each position in the pattern is processed. If the character is '0' or '1', append it and recurse. If it is a wildcard (like '?'), branch into two recursive calls: one appending '0' and one appending '1'. Mention base cases where the current string length matches the pattern length. Emphasize pruning if constraints exist and discuss time complexity based on the number of wildcards.
Key Points to Cover
- Use recursion to explore all branching possibilities
- Handle fixed characters versus wildcards differently
- Define clear base cases for termination
- Manage string concatenation efficiently
Sample Answer
I would implement a recursive backtracking solution. The function takes the current index and the partial string built so far. If the current character in the pattern is '0' or '1', I append it and move to the next index. If it is a wildcard, I make two recursive calls: one appending '0' and another appending '1'. When the index reaches the end of the pattern, I add the complete string to my result list. This ensures all valid permutations are generated efficiently.
Common Mistakes to Avoid
- Missing base cases causing infinite recursion
- Inefficient string concatenation leading to high memory usage
- Forgetting to handle all wildcard variations
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