foo.cc: In function ‘int main()’:
foo.cc:8:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    8 |         for(int i=0;i<s.size();i++)
      |                     ~^~~~~~~~~
foo.cc:10:29: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   10 |                 if(s[i]>='0'&&s[i]<='9'||s[i]>='A'&&s[i]<='z')