##QBIC, 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$. That brings the code for this challenge down to 22 bytes 19 bytes since I've made the cast-to-string autotrim!
:[a|[b,1,-1|Z=Z+!c$