/in/foo.cc: In function 'std::__cxx11::string sub(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:31:5: warning: this 'while' clause does not guard... [-Wmisleading-indentation]
     while(!na[--lmax]&&lmax>0)  ;lmax++;
     ^~~~~
/in/foo.cc:31:34: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'while'
     while(!na[--lmax]&&lmax>0)  ;lmax++;
                                  ^~~~
/in/foo.cc: In function 'std::__cxx11::string div(std::__cxx11::string, int)':
/in/foo.cc:53:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++)
                 ~^~~~~~~~~
/in/foo.cc:59:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<r.size();i++)
                 ~^~~~~~~~~
/in/foo.cc: In function 'int mod(std::__cxx11::string, int)':
/in/foo.cc:66:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++) d=(d*10+(a[i]-'0'))%b;//求出余数
                 ~^~~~~~~~~