给定一个字符串集合，返回字符串中不包含重复字符的字符串集合

 1     /**
 2      * @param arr 给定一个字符串集合
 3      * @return 返回字符串中不包含重复字符的字符串集合
 4      */
 5     public List<String> findStr(List<String> arr) {
 6         List<String> ans = new ArrayList<>();
 7         for (String s : arr) {
 8             int mask = 0;
 9             for (int i = 0; i < s.length(); ++i) {
10                 int ch = s.charAt(i) - ‘a‘;
11                 if (((mask >> ch) & 1) != 0) {
12                     mask = 0;
13                     break;
14                 }
15                 mask |= 1 << ch;
16             }
17             if (mask > 0) {
18                 ans.add(s);
19             }
20         }
21         return ans;
22     }

输入：arr = ["unn","iq","ue","cha"]
输出：arr = ["iq","ue","cha"]
"unn"中包含重复字符‘n‘