string plus enumeration Pattern
4 problems
Pattern pages help build reusable solving frames. Identify signals first, then explain state, transition, and edge handling.
Recognition Signals
- They point out that the expression length is tiny, which is a strong signal to enumerate all parenthesis placements.
- They ask how multiplication appears after adding parentheses, which hints that outside digits become prefix and suffix multipliers.
- Do you recognize that each '?' position is independent within hour and minute limits?
Solve Flow
- 1. Define the active state/window.
- 2. Update state while preserving invariants.
- 3. Validate with edge-heavy examples.
Common Misses
- Forgetting that an empty prefix or empty suffix should contribute 1, not 0, which breaks cases like (999+999).
- Replacing '?' without checking hour > 23 leads to invalid counts.
- Failing to check the bounds of valid hours and minutes when replacing '?' characters.
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