c-gcc, 105 bytes

Result

AC

Code [DL]

char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}while(10-i++){while(s[i]--)printf("%d",i);}}

stdin

3418536745882721739512049921072796670482768537639993168809486584933734839865810949109490309726239812

stdout

0000000011111111222222222333333333333444444444555555666666666777777777788888888888889999999999999999

stderr

/volume/CODE.c:1:20: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}while(10-i++){while(s[i]--)printf("%d",i);}}
                    ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:38: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}while(10-i++){while(s[i]--)printf("%d",i);}}
                                      ^
/volume/CODE.c:1:83: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}while(10-i++){while(s[i]--)printf("%d",i);}}
                                                                                   ^
/volume/CODE.c:1:89: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
 char c,s[10],i=100;main(){while(i--){read(0,&c,1);s[c-48]++;}while(10-i++){while(s[i]--)printf("%d",i);}}
                                                                                         ^
/volume/CODE.c:1:89: warning: incompatible implicit declaration of built-in function ‘printf’
/volume/CODE.c:1:89: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’

strace