[LeetCode] Permutations

Thought:
Recursion.

Code:
public class Solution {
    public ArrayList<ArrayList<Integer>> permute(int[] num) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> cache = new ArrayList<Integer>();
        if (num.length == 1) {
            cache.add(num[0]);
            result.add(cache);
            return result;
        }       
        int first = num[0];
        int[] previous = Arrays.copyOfRange(num, 1, num.length);
        ArrayList<ArrayList<Integer>> temp = permute(previous);
        for (ArrayList<Integer> list : temp) {
            for (int i = 0; i <= list.size(); i++) {
                list.add(i, first);
                result.add(new ArrayList<Integer>(list));
                list.remove(i);
            }
        }
        return result;
    }
}