foo.cc: In function 'void bfs()':
foo.cc:45:39: 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)) {
                                      ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务