foo.cc: In function 'std::vector<std::vector<int> > operate(std::vector<std::vector<int> >, std::__cxx11::string)':
foo.cc:32:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.length(); ++i) {
                     ~~^~~~~~~~~~~~
foo.cc: In function 'bool minPath(std::vector<std::vector<int> >)':
foo.cc:57:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (j = 0; j < op[i].length(); ++j) {
                     ~~^~~~~~~~~~~~~~~~
foo.cc:64:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (j == op[i].length()) {
             ~~^~~~~~~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:89:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < path.length() - 1; ++i) {
                     ~~^~~~~~~~~~~~~~~~~~~

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