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 ArrayList<ArrayList<Integer>> ans = new ArrayList<>();

    public ArrayList<ArrayList<Integer>> permute (int[] num) {
        ArrayList<Integer> track = new ArrayList<>();
        boolean[] used = new boolean[num.length];
        backtrack(num,track,used);
        return ans;

    }

    private void backtrack(int[] num,ArrayList<Integer> track,boolean[] used){
        if(track.size()==num.length){
            ans.add(new ArrayList<>(track));
            return;
        }

        for(int i=0;i<num.length;i++){
            if(used[i]){
                continue;
            }
            track.add(num[i]);
            used[i]=true;
            backtrack(num,track,used);
            track.remove(track.size()-1);
            used[i]=false;
        }
    }