foo.c: In function 'main':
foo.c:33:14: error: expected expression before ')' token
  for(;n<4;n==)
              ^
foo.c:35:3: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   {gets(str[n]);}
   ^
In file included from /usr/include/stdio.h:936:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:233:1: note: declared here
 gets (char *__str)
 ^
foo.c:35:4: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   {gets(str[n]);}
    ^

自豪的采用 HydroJudger 进行评测(github.com/hydro-dev/HydroJudger)