foo.cc: In function 'int main()':
foo.cc:11:8: 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/c++/7/ext/string_conversions.h:43,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.cc:14:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<strlen(a);i++)
              ~^~~~~~~~~~
foo.cc:15:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   s[a[i]]++;
         ^
foo.cc:16:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<strlen(a);i++)
              ~^~~~~~~~~~
foo.cc:17:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(s[a[i]]>max)
            ^
foo.cc:18:14: warning: array subscript has type 'char' [-Wchar-subscripts]
    max=s[a[i]];
              ^
foo.cc:25:12: warning: array subscript has type 'char' [-Wchar-subscripts]
   if(s[a[i]]==max)
            ^
foo.cc:13:6: warning: unused variable 'cnt' [-Wunused-variable]
  int cnt=0;
      ^~~
foo.cc:11:6: warning: ignoring return value of 'char* gets(char*)', declared with attribute warn_unused_result [-Wunused-result]
  gets(a);
  ~~~~^~~
/tmp/ccGdH1hq.o: In function `main':
foo.cc:(.text.startup+0x45): 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。