foo.cc: In function 'int main()':
foo.cc:16:17: warning: array subscript has type 'char' [-Wchar-subscripts]
         num[a[i]]++;
                 ^
foo.cc:17:20: warning: array subscript has type 'char' [-Wchar-subscripts]
         if(num[a[i]]>max)
                    ^
foo.cc:19:25: warning: array subscript has type 'char' [-Wchar-subscripts]
             max=num[a[i]];
                         ^
foo.cc:24:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++)
                 ~^~~~~~~~~
foo.cc:30:34: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
         if(i<len||a[i]==maxletter&&i==len)
foo.cc:26:30: warning: 'maxletter' may be used uninitialized in this function [-Wmaybe-uninitialized]
         while(a[i]!=maxletter&&i<len)

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