foo.cc: In function 'int main()':
foo.cc:13:10: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
   gets(s1);
          ^
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:13:10: error: cannot convert 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' to 'char*' for argument '1' to 'char* gets(char*)'
   gets(s1);
          ^
foo.cc:14:24: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
                 gets(s2);
                        ^
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:14:24: error: cannot convert 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' to 'char*' for argument '1' to 'char* gets(char*)'
                 gets(s2);
                        ^
foo.cc:15:10: error: expected primary-expression before '=' token
   if(s1!==s2)
          ^

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