/in/foo.cc: In function 'int main()':
/in/foo.cc:41:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<x.length();i++)
              ~^~~~~~~~~~~
/in/foo.cc:47:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(tar>=x.length()-1){
     ~~~^~~~~~~~~~~~~~
/in/foo.cc:49:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=1;i<=tar-x.length()+1;i++)
               ~^~~~~~~~~~~~~~~~~~
/in/foo.cc:46:14: warning: 'pos' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int tar=pos+k;//保留到的位数 
              ^