foo.cc: In member function 'void Solution::read()':
foo.cc:40:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (size_t i = 0; i < fl; i++)
                        ~~^~~~
foo.cc:45:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (size_t i = 0; i < sl; i++)
                        ~~^~~~