you can use set also:

    Scanner scan = new Scanner(System.in);
    String str = scan.nextLine();

    Set<Character> set = new HashSet<Character>();
    for(Character ch : str.toCharArray()){
        if(set.contains(ch)){
            set.remove(ch);
        }else{
            set.add(ch);
        }
    }

    System.out.println((set.size()<=1)?"YES":"NO");


    scan.close();