foo.cc: In function 'void bfs()':
foo.cc:47:33: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for(int j=t.s.find(a[i]);j>=0&&j<=(t.s.length()-a[i].length());j=t.s.find(a[i],j+1))
                                ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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