foo.cc: In function 'int main()':
foo.cc:30:35: warning: comparison of integer expressions of different signedness: 'long long int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   30 |     for (int i=0,j=s.length()-1; i<s.length()&&i<=j; i++,j--) {
      |                                  ~^~~~~~~~~~~

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