2026PKUCourseHW5: Replace fixed-size char array with std::string

[GenZeal-Lin]

Unit Tests and/or Case Tests for my changes

• No new unit test was added.
• The modified code path was manually reviewed.
• The change replaces a fixed-size character buffer with std::string to avoid potential buffer overflow risk.

What's changed?

• Replace a fixed-size char array used for string input with std::string.
• Keep the original logic unchanged while making the string handling safer and clearer.