foo.cc: In function 'int cal(std::__cxx11::string, int)':
foo.cc:39:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < expr.size(); i ++ )
                     ~~^~~~~~~~~~~~~
foo.cc:45:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             while (j < expr.size() && expr[j] >= '0' && expr[j] <= '9')
                    ~~^~~~~~~~~~~~~

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