bash-pure, 61 bytes

Result

AC

Code [DL]

while read -n1 c;do a[c]+=$c;done
echo $(IFS=;echo "${a[*]}")

stdin

1734225638130352252106274476679252805047797668172018436374617562004597552846291371668011834398235003

stdout

0000000000111111111122222222222223333333333344444444455555555556666666666667777777777778888888899999

stderr

/bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)

strace