foo.cc: In function 'void bfs()':
foo.cc:73:53: 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))
                                                    ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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