超过时间限制，我正在尝试使用java堆栈使用链表删除字符串中的重复项

import java.util.Stack;

public class RemoveDuplicates {
    public static String removeDuplicates(String str) {
        Stack<Character> stack = new Stack<>();
        StringBuilder result = new StringBuilder();

        for (char c : str.toCharArray()) {
            if (stack.isEmpty()) {
                stack.push(c);
            } else {
                if (stack.peek() == c) {
                    stack.pop();
                } else {
                    stack.push(c);
                }
            }
        }

        while (!stack.isEmpty()) {
            result.insert(0, stack.pop());
        }

        return result.toString();
    }

    public static void main(String[] args) {
        String str = "aabbbccdd";
        String result = removeDuplicates(str);
        System.out.println(result);  // 输出：ad
    }
}