LeetCode 46 - Permutations

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

public List<List<Integer>> permute(int[] num) {
    List<List<Integer>> result = new ArrayList<>();
    perm(result, num, 0);
    return result;
}

public void perm(List<List<Integer>> result, int[] num, int pos) {
    if(pos == num.length) {
        List<Integer> list = new ArrayList<Integer>();
        for(int a:num) list.add(a);
        result.add(list);
        return;
    }
    for(int i=pos; i<num.length; i++) {
        swap(num, i, pos);
        perm(result, num, pos+1);
        swap(num, i, pos);
    }
}

private void swap(int[] num, int i, int j) {
    int tmp = num[i];
    num[i] = num[j];
    num[j] = tmp;
}