foo.cc: In function 'int caozuo(int, int, std::vector<std::vector<int> >&, std::vector<std::vector<int> >&)':
foo.cc:24:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (b < arr[0].size() - 1)
      ~~^~~~~~~~~~~~~~~~~~~
foo.cc:31:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (b2 == arr[0].size() - 1)
        ~~~^~~~~~~~~~~~~~~~~~~~
foo.cc:36:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (a < arr.size() - 1)
      ~~^~~~~~~~~~~~~~~~
foo.cc:44:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (a2 == arr.size() - 1)
        ~~~^~~~~~~~~~~~~~~~~

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