/in/foo.cc: In member function 'bool MinFlowCost::spfa(int, int, int&)':
/in/foo.cc:74:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int j = 0; j < v_flows[from].size(); j++)
                    ~~^~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc: In member function 'int MinFlowCost::Calc(int, int)':
/in/foo.cc:117:8: warning: unused variable 'flow' [-Wunused-variable]
   int  flow = 0;
        ^~~~