输入一个字符串，最终输出的去掉重复后的字符串，保证输入顺序。

String s = removeDuplicates("accdalfcld");
输出 s = "acdlf";
用LinkedHashSet存储，保证了不重复和有序。

public String removeDuplicates(String str){
  if(TextUtils.isEmpty(str)){
    return str;
  }
  int length = str.length();
  if (length == 1) {
    return str;
  }
  LinkedHashSet set = new LinkedHashSet<>();
  for (int i = 0; i < length-1; i++) {
    set.add(str.charAt(i));
  }
  StringBuilder stringBuilder = new StringBuilder();
  for (Character c: set){
    stringBuilder.append(c);
  }
  return stringBuilder.toString();
}