/in/foo.cc: In function 'int main()':
/in/foo.cc:15:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<=x.size();i++)//字符x的长度 
               ~^~~~~~~~~~
/in/foo.cc:16:15: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   if(x[i]=y[i])               //不确定等不能用 
               ^
/in/foo.cc:37:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<z.size();++i)
              ~^~~~~~~~~