foo.cc: In function 'int main()':
foo.cc:17:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   17 |         for(int i=0;i<a.length();i++)
      |                     ~^~~~~~~~~~~
foo.cc:39:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   39 |         for(int i=0;i<a.length();i++)
      |                     ~^~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务