By hand it's a super easy pattern. Effectively you're taking 111,111,111 then multiplying it by each position of 1.

 111,111,111

x 111,111,111

 111,111,111

+) 111,111,111,0
+) 111,111,111,00
+) 111,111,111,000

Etc, repeating 9 times. Add it all together and you receive the answer:

12345678987654321

;)