foo.cc: In function 'int main()':
foo.cc:7:21: error: too few arguments to function 'int putchar(int)'
  while(a[n]=putchar()!="\n")
                     ^
In file included from /usr/include/stdio.h:859:0,
                 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/stdio.h:79:1: note: declared here
 putchar (int __c)
 ^~~~~~~

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