/in/foo.cc: In function 'int Diff(char*)':
/in/foo.cc:5:17: warning: NULL used in arithmetic [-Wpointer-arith]
  while (s[i] != NULL) {
                 ^~~~
/in/foo.cc: In function 'void Concat(char*, char*, char*)':
/in/foo.cc:19:6: warning: unused variable 'i' [-Wunused-variable]
  int i, k = 0;
      ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:41:2: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(s1);
  ^~~~
In file included from /in/foo.cc:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.cc:41:9: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(s1);
         ^
In file included from /in/foo.cc:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/tmp/ccMCc1ax.o: In function `main':
foo.cc:(.text.startup+0x11): warning: the `gets' function is dangerous and should not be used.