Permutations

class Solution {
    /**
     * @param nums: A list of integers.
     * @return: A list of permutations.
     */
    public ArrayList<ArrayList<Integer>> permute(ArrayList<Integer> nums) {
        // write your code here
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(nums == null || nums.size() == 0){
            return result;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        permuteHelper(result, list, nums, new boolean[nums.size()]);
        return result;
    }
    
    private void permuteHelper(ArrayList<ArrayList<Integer>> result,
                          ArrayList<Integer> list, 
                          ArrayList<Integer> nums,
                          boolean[] used){
        if (list.size() == nums.size()){
            result.add(new ArrayList<Integer>(list));
            return;
        }
        for(int i = 0; i < nums.size(); i++){
            if(used[i]){
                continue;
            }
            used[i] = true;
            list.add(nums.get(i));
            permuteHelper(result, list, nums, used);
            list.remove(list.size() - 1);
            used[i] = false;
        }
    }
}