String 与 char 互转

对String里的char进行处理过滤再转回String

考虑了大于char表示范围的符号

char 2个byte，有的字符会占3、4或者更多byte。

https://stackoverflow.com/questions/31977356/using-streams-to-manipulate-a-string

StringBuilder sb = ... ;
string.codePoints()
      .filter(...)
      .forEachOrdered(sb::appendCodePoint);
return sb.toString();

对于字母

private Object encode1(String str) {
    StringBuilder sb = new StringBuilder("");
    str.chars().sorted().map(c -> (char)c).forEach(sb::appendCodePoint);
    return sb.toString();
}

https://leetcode.com/problems/group-anagrams/solution/

char[] ca = s.toCharArray();
Arrays.sort(ca);
String key = String.valueOf(ca);