foo.cc: In function 'int main()':
foo.cc:10:2: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
  if(n==sx(n))    cout<<"YES";
  ^~
foo.cc: In function 'int sx(int)':
foo.cc:23:9: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
  return sum;
         ^~~

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