foo.cc: In function 'bool SPFA(int, int)':
foo.cc:91:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int z = 0; z < edge[x].size(); z++) {
                         ~~^~~~~~~~~~~~~~~~
foo.cc: In function 'newpair ZKW_DFS(int, int, int)':
foo.cc:146:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int& z = head[x]; z < edge[x].size(); z++) {
                            ~~^~~~~~~~~~~~~~~~
foo.cc: In function 'void costflow()':
foo.cc:174:20: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
         while (ans = ZKW_DFS(0, st, inf)) {
                ~~~~^~~~~~~~~~~~~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。