遞歸練習一：輸入一個字符串，打印出該字符串中字符的所有排列

例子

例如輸入字符串abc，則輸出由字符a、b、c所能排列出來的所有字符串abc、acb、bac、bca、cab和cba。
java代碼

 public void sort(List<Character> oldChars, int k, List<String> res) {
        if (k == oldChars.size() - 1){//輸出這個排列
            String resTemp = "";
            for (int i = 0; i < oldChars.size(); i++) {
                System.out.print(oldChars.get(i) + " ");
                resTemp += oldChars.get(i);
                }
            res.add(resTemp);
            System.out.println();
          }
        for (int i = k; i < oldChars.size(); i++) {
            System.out.println("走進for循環中 i=" + i);
            Character temp = oldChars.get(i);
            oldChars.set(i, oldChars.get(k));
            oldChars.set(k, temp);
            sort(oldChars, k+1, res);
            //再換回來
            Character temp1 = oldChars.get(i);
            oldChars.set(i, oldChars.get(k));
            oldChars.set(k, temp1);
        }
    }