private static void fun(){
     System.out.println("请输入需要检测的字符串:");
    //控制台输出语句
    Scanner sc = new Scanner(System.in); 
   while (sc.hasNext()){
        String str = sc.nextLine();
        System.out.println(retStr(str)); 
   }   
 sc.close();
}

private  static String retStr(String s){
    //遍历字符串,并计算字符出现的次数
    for (int i = 0; i < s.length(); i++ ){
        String sub = s.substring(i,i+1);
        if ((s.length() - s.replaceAll(sub,"").length() == 1)){
            return sub;
        }
    }
    return ".";
}