/in/foo.cc: In member function 'bool Sudoku::is_solved() const':
/in/foo.cc:49:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int k = 0; k < _cells.size(); k++) {
                  ~~^~~~~~~~~~~~~~~
/in/foo.cc: In member function 'void Sudoku::write(std::ostream&) const':
/in/foo.cc:59:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int k = 0; k < _cells.size(); k++) {
                  ~~^~~~~~~~~~~~~~~
/in/foo.cc: In static member function 'static void Sudoku::init()':
/in/foo.cc:89:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int k = 0; k < _neighbors.size(); k++) {
                  ~~^~~~~~~~~~~~~~~~~~~
/in/foo.cc:90:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int x = 0; x < _groups_of[k].size(); x++) {
                   ~~^~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc: In member function 'bool Sudoku::eliminate(int, int)':
/in/foo.cc:119:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < _neighbors[k].size(); i++) {
                   ~~^~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:123:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < _groups_of[k].size(); i++) {
                  ~~^~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc: In member function 'int Sudoku::least_count() const':
/in/foo.cc:146:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < _cells.size(); i++) {
                  ~~^~~~~~~~~~~~~~~
/in/foo.cc: In constructor 'Sudoku::Sudoku(std::__cxx11::string)':
/in/foo.cc:159:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size(); i++) {
                  ~~^~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:195:6: warning: unused variable 'n' [-Wunused-variable]
  int n = 0;
      ^
/in/foo.cc: In member function 'int Sudoku::least_count() const':
/in/foo.cc:148:30: warning: 'min' may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (m > 1 && (k == -1 || m < min)) {
                            ~~^~~~~