java 字符串排列组合

public static LinkedList<String> permutation(String str) {                LinkedList<String> linkedString = new LinkedList<String>();           if (str.length() <= 1) {              linkedString.add(str);              return linkedString;          }           for (int i = 0; i < str.length() ; i++) {              char ch = str.charAt(i);              //consider the case in which the characters may be duplicated.              if (i > 0 && ch == str.charAt(i - 1)) {                  continue;              }              String newStr = remove(str, i);              LinkedList<String> newStrList= permutation(newStr);               for (int j = 0; j < newStrList.size(); j++) {                  linkedString.add(ch + newStrList.get(j));              }          }          return linkedString;      }      //remove the ith character from the string      public static String remove(String str, int i) {          if (i == 0) return str.substring(1, str.length());          if (i == str.length() - 1) return str.substring(0, i );          return str.substring(0, i) + str.substring(i + 1, str.length());      }