字符串全排列

输入一个字符串,按字典序打印出该字符串中字符的所有排列。例如输入字符串abc,则打印出由字符a,b,c所能排列出来的所有字符串abc,acb,bac,bca,cab和cba。

private static ArrayList<String> ret = new ArrayList<>();
    public static ArrayList<String> Permutation(String str){
        if (str.length() == 0){
            return ret;
        }
        char[] chars = str.toCharArray();
        Arrays.sort(chars);
        backtracking(chars, new boolean[chars.length], new StringBuilder());
        return ret;
    }
    private static void backtracking(char[] chars, boolean[] hasUsed, StringBuilder s){
        if (s.length() == chars.length){
            ret.add(s.toString());
            return;
        }
        for (int i = 0; i < chars.length; i++){
            if (hasUsed[i]){
                continue;
            }
            if (i != 0 && chars[i] == chars[i - 1] && !hasUsed[i - 1]){
                continue;
            }
            hasUsed[i] = true;
            s.append(chars[i]);
            backtracking(chars, hasUsed, s);
            s.deleteCharAt(s.length() - 1);
            hasUsed[i] = false;
        }
    }