foo.cc: In function 'int query(int, int, int)':
foo.cc:47:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0;i < pre.size();++i) {
                    ~~^~~~~~~~~~~~
foo.cc:54:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0;i < pre.size();++i) {
                    ~~^~~~~~~~~~~~