foo.cpp: In function 'int dfs(int, int)':
foo.cpp:44:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < vc[cur].size(); ++i) {
                     ~~^~~~~~~~~~~~~~~~
foo.cpp: In function 'void work()':
foo.cpp:83:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < res.size(); ++i) {
                     ~~^~~~~~~~~~~~
foo.cpp:67:9: warning: unused variable 'root' [-Wunused-variable]
     int root = 1;
         ^~~~