字符串的排列

import java.util.ArrayList;

public class Solution {
    private int length = 0;

    public ArrayList<String> Permutation(String str) {
        length = str.length();

        return per(str);
    }

    private ArrayList<String> per(String str) {
        ArrayList<String> list = new ArrayList<>();
        if (str.length() == 1) {
            list.add(str);
        }
        else {
            for (int i=0; i<str.length(); i++) {
                String subStr = str.substring(0, i) + str.substring(i+1, str.length());
                ArrayList<String> subList = per(subStr);
                for (String s : subList) {
                    String newStr = str.substring(i,i+1) + s;
                    if (newStr.length() == length && list.contains(newStr))
                        continue;
                    list.add(newStr);
                }
            }
        }
        return list;
    }
}