c-gcc, 102 bytes

Result

AC

Code [DL]

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

stdin

6005161377868731105628628885260311707454454009788218422555670578570913257379482043372853015914555624

stdout

0000000000011111111112222222222333333334444444445555555555555555666666667777777777778888888888889999

stderr

/volume/CODE.c:1:21: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char c,s[100],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                     ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:39: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char c,s[100],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                       ^
/volume/CODE.c:1:53: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[100],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                     ^
/volume/CODE.c:1:81: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char c,s[100],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                                                 ^
/volume/CODE.c:1:87: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
 char c,s[100],i=100;main(){while(i--){read(0,&c,1);s[c]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                                                       ^

strace