feat: let KMP algorithm return index

[Yancey2023]

Description of Change

For strings/knuth_morris_pratt.cpp:

fix: if pattern is empty, it should immediately return, otherwise pattern[k] will access out of bounds.

feat: let the KMP algorithm return index and add more tests.

Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes:

[realstealthninja]

If you have the time could you write tests for this algorithm?

[Yancey2023]

Hi, I've added some test cases! Additionally, I've changed the return value to the starting index of the pattern. Perhaps the usage of std::string::npos and size_t in the code might confuse beginners, but this is based on my reference to std::string::find(), which indeed returns a size_t type and returns std::string::npos when the substring is not found.

[Yancey2023]

I don't understand:

#ifdef _MSC_VER
#include <string>  // use this for MS Visual C++
#else
#include <cstring>
#endif

[Yancey2023]

I don't understand:

#ifdef _MSC_VER
#include <string>  // use this for MS Visual C++
#else
#include <cstring>
#endif

Why not just use #include <string>?

[realstealthninja]

It's old code feel free to remove

[Yancey2023]

Thanks. I have deleted it.