如何根据字符串的组合删除重复项？

import java.util.HashSet;

public class RemoveDuplicates {
    public static String removeDuplicates(String str) {
        char[] chars = str.toCharArray();
        StringBuilder result = new StringBuilder();
        HashSet<Character> set = new HashSet<>();

        for (char c : chars) {
            if (!set.contains(c)) {
                set.add(c);
                result.append(c);
            }
        }

        return result.toString();
    }

    public static void main(String[] args) {
        String input = "aabbccdd";
        String output = removeDuplicates(input);
        System.out.println(output);  // 输出: abcd
    }
}