foo.cc: In function 'int main()':
foo.cc:19:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   19 |     for(int i=0;i<a.size();i++)
      |                 ~^~~~~~~~~
foo.cc:28:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   28 |     for(int i=0;i<a.size()/2;i++)
      |                 ~^~~~~~~~~~~
foo.cc:15:9: warning: unused variable 'flag' [-Wunused-variable]
   15 |     int flag=1;
      |         ^~~~

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