c-gcc, 99 bytes

Result

WA

Code [DL]

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

stdin

6300847183421775095843830174456770611248433205709369431388726723751520869985977938630654936161843813

stdout

0000000001111111111222222333333333333333444444444455555555666666666677777777777788888888888899999999

stderr

/volume/CODE.c:1:17: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char s[1],i=100;main(){while(i--){read(0,&s,1);s[*s]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                 ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:35: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char s[1],i=100;main(){while(i--){read(0,&s,1);s[*s]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                   ^
/volume/CODE.c:1:49: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char s[1],i=100;main(){while(i--){read(0,&s,1);s[*s]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                 ^
/volume/CODE.c:1:78: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char s[1],i=100;main(){while(i--){read(0,&s,1);s[*s]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                                              ^
/volume/CODE.c:1:84: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
 char s[1],i=100;main(){while(i--){read(0,&s,1);s[*s]++;}while(58-i++){while(s[i]--)write(1,&i,1);}}
                                                                                    ^

strace