/in/foo.cc: In function 'void dfs(int)':
/in/foo.cc:30:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < vv[u].size(); ++ i) {
                     ~~^~~~~~~~~~~~~~
/in/foo.cc:36:33: warning: suggest parentheses around comparison in operand of '^' [-Wparentheses]
             if (vis[v] ^ vis[u] != 1) {
                          ~~~~~~~^~~~