foo.cc: In function 'int main()':
foo.cc:28:18: warning: 's1' is used uninitialized in this function [-Wuninitialized]
  for(int i=0; a[i]!=0 && a[i+1]!=0; i++)
               ~~~^
foo.cc:28:31: warning: '*((void*)& s1 +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
  for(int i=0; a[i]!=0 && a[i+1]!=0; i++)
                          ~~~~~^
foo.cc:28:18: warning: 's2' is used uninitialized in this function [-Wuninitialized]
  for(int i=0; a[i]!=0 && a[i+1]!=0; i++)
               ~~~^
foo.cc:28:31: warning: '*((void*)& s2 +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
  for(int i=0; a[i]!=0 && a[i+1]!=0; i++)
                          ~~~~~^

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