感觉中间用数组方便些 最后用stream来转化
用时也少一些

class Solution {
    static int[] root;
    static HashSet<List<Integer>> res = new HashSet<>();
    public static List<List<Integer>> permutation(int[] nums) {
        root = nums;
        boolean[] tmp = new boolean[root.length];
        dfs(new int[root.length],0,tmp);
        return res.stream().collect(Collectors.toList());
    }
    public static void dfs(int[] a,int id,boolean[] used){
        if (id == root.length) {
         res.add(Arrays.stream(a).boxed().collect(Collectors.toList()));
        }
        for (int i = 0; i < root.length; ++i){
            if (!used[i]){
                used[i] = true;
                a[id] = root[i];
                dfs(a,id+1,used);
                used[i] = false;
            }
        }
    }
}