Permutation

public class Solution {
    public ArrayList<ArrayList<Integer>> permute(int[] num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(num.length == 0) return null;
        ArrayList<Integer> container = new ArrayList<Integer>();
        Integer[] temp = new Integer[num.length];
        for(int i=0; i<num.length; i++) temp[i] = num[i];
        return helpler(temp, 0, temp.length-1);
    }
    private ArrayList<ArrayList<Integer>> helpler(Integer[] num, int start, int end){
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(start > end) result.add(new ArrayList<Integer>(Arrays.asList(num)));
        else{
            for(int i=start; i<=end; i++)
            {
                swap(num, start, i);
                result.addAll(helpler(num, start+1, end));
                swap(num, start, i);
            }
        }
        return result;
    }
    private void swap(Integer[] num, int a, int b){
        int temp = num[a];
        num[a] = num[b];
        num[b] = temp;
    }
}