/in/foo.cc:15:8: warning: multi-character character constant [-Wmultichar]
  s1[k]='/0';
        ^~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:7:2: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(s);
  ^~~~
In file included from /usr/include/c++/6/cstdio:42:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:46,
                 from /in/foo.cc:1:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.cc:7:8: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(s);
        ^
In file included from /usr/include/c++/6/cstdio:42:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:46,
                 from /in/foo.cc:1:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.cc:10:17: warning: for increment expression has no effect [-Wunused-value]
  for(i=n-1;i>=0;i)
                 ^
/in/foo.cc:15:8: warning: overflow in implicit constant conversion [-Woverflow]
  s1[k]='/0';
        ^~~~
/tmp/cct407eh.o: In function `main':
foo.cc:(.text.startup+0xf): warning: the `gets' function is dangerous and should not be used.