## [QBIC](https://drive.google.com/drive/folders/0B0R1Jgqp8Gg4cVJCZkRkdEthZDQ), 25 bytes

    :[a|[b,1,-1|A=A+_t!c$|}?A

Sample run:

    Command line: 10
    12132143215432165432176543218765432198765432110987654321

Explanation:

    :           Get 'a' from the command line
    [a|         FOR b=1; b<=a; b++
    [b,1,-1|      FOR c=b; c>=1; c--
    A=A+_t!c$|      Append c to the 'output buffer' A$
    }           Close the FOR loops
    ?A          Print 'a'

Non-competing: five minutes ago, I taught QBIC to do implicit printing on exit of anything stored in `Z$`. That brings the code for this challenge down to <s>22 bytes</s> 19 bytes since I've made the cast-to-string autotrim!

    :[a|[b,1,-1|Z=Z+!c$