foo.cc: In function 'void beginfrom(int, int)':
foo.cc:15:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < vec[beg].size(); i++)
                  ~~^~~~~~~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:49:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int k = 0; k < vec[j].size(); k++)
                     ~~^~~~~~~~~~~~~~~

自豪的采用jd5进行评测(github.com/masnn/jd5)