/in/foo.cc: In function 'int f(int)':
/in/foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a[x].size();j++){
              ~^~~~~~~~~~~~
/in/foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a[x].size();j++){
              ~^~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:43:10: warning: 'root' may be used uninitialized in this function [-Wmaybe-uninitialized]
   f(root);
          ^