强制转换不适用于嵌套列表对象类型并返回空列表 (List>)

我正在做一些与回溯相关的 leetcode 挑战，即:https://leetcode.com/problems/permutations/

I'm doing some leetcode challenges related to backtracking, namely: https://leetcode.com/problems/permutations/

我需要返回 List> 作为类型的地方，但是我的 List> 只有在我接受 List 作为我的参数，然后将其转换为 ArrayList，同时将其添加到主要结果中.

Where I need to return List<List<Integer>> as the type, however my List<List<Integer>> only gets populated correctly if I accept List<Integer> as my parameter and I cast it to ArrayList<Integer> while I add it in the main result.

代码:

    List<List<Integer>> result = new ArrayList<>();
    public List<List<Integer>> permute(int[] nums) {
        bt(new ArrayList<Integer>(), nums);
        return result;
    }
    
    public void bt(List<Integer> tmp, int[] nums){
        if(tmp.size() == nums.length){
            result.add(new ArrayList<>(tmp));  // HERE
        }else{
          // some logic here
            
        }
    }

输出:[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

Output: [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

对比下面的代码，我接受 ArrayList 作为参数

代码:

List<List<Integer>> result = new ArrayList<>();   
public List<List<Integer>> permute(int[] nums) {
    bt(new ArrayList<Integer>(), nums);
    return result;
}
public void bt(ArrayList<Integer> tmp, int[] nums){
    if(tmp.size() == nums.length){
        result.add(tmp); // HERE
    }else{
      ... some logic here
    
    }
}

输出: [[],[],[],[],[],[]]

Output: [[],[],[],[],[],[]]