c-gcc, 98 bytes

Result

AC

Code [DL]

char c,s[1],i=100;main(){for(;i--;s[c]++)read(0,&c,1);for(;58-i++;){for(;s[i]--;write(1,&i,1));};}

stdin

0216809016015925640917814989513903856716243236993663230032137661956676960721588088381834574408897349

stdout

0000000000011111111111222222233333333333344444445555555666666666666667777777888888888888999999999999

stderr

/volume/CODE.c:1:19: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char c,s[1],i=100;main(){for(;i--;s[c]++)read(0,&c,1);for(;58-i++;){for(;s[i]--;write(1,&i,1));};}
                   ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:36: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[1],i=100;main(){for(;i--;s[c]++)read(0,&c,1);for(;58-i++;){for(;s[i]--;write(1,&i,1));};}
                                    ^
/volume/CODE.c:1:42: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char c,s[1],i=100;main(){for(;i--;s[c]++)read(0,&c,1);for(;58-i++;){for(;s[i]--;write(1,&i,1));};}
                                          ^
/volume/CODE.c:1:75: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[1],i=100;main(){for(;i--;s[c]++)read(0,&c,1);for(;58-i++;){for(;s[i]--;write(1,&i,1));};}
                                                                           ^
/volume/CODE.c:1:81: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
 char c,s[1],i=100;main(){for(;i--;s[c]++)read(0,&c,1);for(;58-i++;){for(;s[i]--;write(1,&i,1));};}
                                                                                 ^

strace