string·结合·enumeration 模式

识别信号

• 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?

解题流程

1. 1. 明确窗口/状态定义
2. 2. 更新状态并维护约束
3. 3. 用边界样例验证

常见失分点

• 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.