ruby2.4, 20 bytes

Result

AC

Code [DL]

$><<$<.chars.sort*''

stdin

6131964941979599223374889023023766160401527580985962233195431853064926201953730792369567106470350670

stdout

0000000000001111111112222222222333333333333344444445555555556666666666667777777778888899999999999999

stderr

/volume/CODE:1: warning: ARGF#chars is deprecated; use #each_char instead

strace