public String getPermutation(int n, int k) {
        //itemlist 為原始順序數列
        List<Integer> itemList = new ArrayList<>();
        if(n==1){
            return "1";
        }
        //階乘資料
        List<Integer> nList = new ArrayList<>();
        nList.add(1);
        for(int i=1;i<=n;i++){
            itemList.add(i);
            
 
if(i>=2){
                //計算階乘值 N! = n* (N-1)!
                int nData = nList.get(i-2);
                nList.add(i*nData);
            }
        }

        if(k > nList.get(nList.size()-1)){
            return "";
        }

        int index = n-1;

        k = k-1;

        StringBuilder itemStr 
 
= new StringBuilder();

        while(index>=0){
            if(index ==0){
                itemStr.append(itemList.get(0));
                break;
            }
            //獲取N-1的階乘值
            int result = nList.get(index-1);
            //獲得當前薰迴圈的第一個數字的索引
            int firstIndex = k/result;
            
 
//獲取當前第一個數字
            int firstNum = itemList.get(firstIndex);
            itemStr.append(firstNum);
            itemList.remove(firstIndex);
            index = index -1;
            k = k%result;
        }
        return itemStr.toString();
    }

——2020.7.10