/in/foo.cc: In function 'int dfs(treenode*)':
/in/foo.cc:29:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int k = 0; k < head->sons.size(); ++k) {
                     ~~^~~~~~~~~~~~~~~~~~~
/in/foo.cc: In function 'void bfs(treenode*)':
/in/foo.cc:40:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int k = 0; k < head->sons.size(); ++k) {
                     ~~^~~~~~~~~~~~~~~~~~~
/in/foo.cc:46:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int k = 0; k < head->sons.size(); ++k) {
                     ~~^~~~~~~~~~~~~~~~~~~
/in/foo.cc:39:29: warning: unused variable 'ans' [-Wunused-variable]
     int maxx = 0, idx = -1, ans = 0;
                             ^~~