foo.cc: In function 'int Substring(std::__cxx11::string)':
foo.cc:8:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < str.size(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:9:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = 1; j <= ((str.substr(i)).size()); j++) 
                         ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

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