Golang

package main

import (
    "bufio"
    "fmt"
    "os"
    "sort"
    "strings"
)

func main() {
    sc := bufio.NewScanner(os.Stdin)
    for sc.Scan() {
        strs:=strings.Split(sc.Text(),",")
        sort.Strings(strs)
        fmt.Println(strings.Join(strs,","))
    }
}
/*input
a,c,bb
f,dddd
nowcoder
*/