/in/foo.cc: In member function 'bool MaxFlowCost::spfa(int, int, int&)':
/in/foo.cc:94:22: 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 MaxFlowCost::Calc(int, int)':
/in/foo.cc:130:7: warning: unused variable 'flow' [-Wunused-variable]
   int flow = 0;
       ^~~~