What is the best way to generate binary strings from a pattern?
Candidates must generate all possible binary strings that match a specific pattern containing wildcards or fixed characters. This tests recursion, backtracking, and string manipulation skills.
Why Interviewers Ask This
This question evaluates a candidate's grasp of recursive backtracking algorithms. Interviewers want to see if the candidate can systematically explore all possibilities without missing any combinations. It also tests their ability to manage state during recursion and prune invalid branches efficiently.
How to Answer This Question
Explain the recursive function structure where you process the pattern character by character. If the character is a wildcard, branch into two paths (0 and 1). If it is fixed, proceed directly. Base case occurs when the current string length matches the pattern length. Mention pruning strategies if the pattern has constraints.
Key Points to Cover
- Use recursion to explore branching possibilities
- Handle wildcards by creating multiple recursive calls
- Base case triggers when pattern length is reached
- Store valid results in a collection
Sample Answer
I would use a recursive backtracking approach. The function takes the current index and the partially built string. If the current character in the pattern is '?', I recursively call the function twice: once appending '0' and once appending '1'. If the character is '0' or '1', I append it and move to the next index. When the index reaches the pattern length, I add the completed string to my result list. This ensures all valid combinations are generated.
Common Mistakes to Avoid
- Forgetting to reset state between recursive branches
- Infinite recursion due to missing base cases
- Incorrectly handling fixed characters versus wildcards
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