foo.cc: In function 'int f(int)':
foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a[x].size();j++){
              ~^~~~~~~~~~~~
foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a[x].size();j++){
              ~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:26:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d",&k,&n);
  ~~~~~^~~~~~~~~~~~~~
foo.cc:31:9: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d%d",&x,&y);
    ~~~~~^~~~~~~~~~~~~~
foo.cc:42:13: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
   ans[i] = f(root);
            ~^~~~~~

自豪的采用 HydroJudge 进行评测(github.com/hydro-dev/HydroJudge)

{"receive":"2020-07-21T12:51:40.637Z","handle":"2020-07-21T12:51:40.637Z","cache_start":"2020-07-21T12:51:40.644Z","read_cases":"2020-07-21T12:51:40.773Z","judge":"2020-07-21T12:51:40.773Z","done":"2020-07-21T12:51:41.313Z"}