foo.cc: In function 'int main()':
foo.cc:6:8: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(a);
        ^
In file included from /usr/include/stdio.h:862:0,
                 from /usr/include/c++/7/cstdio:42,
                 from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:46,
                 from foo.cc:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.cc:9:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   b[a[i]]++;
         ^
foo.cc:12:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(b[a[i]]>max)max=b[a[i]];
            ^
foo.cc:12:28: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(b[a[i]]>max)max=b[a[i]];
                            ^
foo.cc:15:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(b[a[i]]==max)cout<<b[a[i]];
            ^
foo.cc:15:31: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(b[a[i]]==max)cout<<b[a[i]];
                               ^
foo.cc:6:6: warning: ignoring return value of 'char* gets(char*)', declared with attribute warn_unused_result [-Wunused-result]
  gets(a);
  ~~~~^~~
/tmp/cckzL2XH.o: In function `main':
foo.cc:(.text.startup+0x2e): warning: the `gets' function is dangerous and should not be used.

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