d-gdc, 114 bytes

Result

AC

Code [DL]

import std.stdio;import std.array;import std.algorithm;void main(){write(join(sort!("a<b")(split(readln(),""))));}

stdin

9690248341339541315464463154976994992009828843358682748225172007084778904738740827932915867713444598

stdout

0000000011111112222222223333333333444444444444444455555556666667777777777788888888888889999999999999

stderr

strace