/in/foo.cc: In function 'void f(int, int)':
/in/foo.cc:13:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int k=0;k<to[x].size();k++)
              ~^~~~~~~~~~~~~
/in/foo.cc: In function 'void c(int, int)':
/in/foo.cc:20:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int k=0;k<to[x].size();k++)
              ~^~~~~~~~~~~~~
/in/foo.cc: In function 'void pr(int)':
/in/foo.cc:27:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int l=0;l<tree[k].size();l++)
               ~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'void dfs(int, int)':
/in/foo.cc:42:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int k=0;k<tree[x].size();k++)
              ~^~~~~~~~~~~~~~~
/in/foo.cc:46:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int l=0;l<to[tree[x][k]].size();l++)
               ~^~~~~~~~~~~~~~~~~~~~~~