要求实现字符逆序。

解析：

String str = "ABC123ABC";StringBuilder stringBuilder = new StringBuilder(str);
stringBuilder.reverse();String newStr = stringBuilder.toString();System.out.println("反转前：" + str);
System.out.println("反转后：" + newStr);String str = "ABC123ABC";char[] charArray = str.toCharArray();
Stack stack = new Stack<>();
StringBuilder newStr = new StringBuilder();for (char item : charArray) {stack.push(item);
}for (int i = 0; i < charArray.length; i++) {newStr.append(stack.pop());
}System.out.println("反转前：" + str);
System.out.println("反转后：" + newStr.toString());