Permutations

Given a collection of distinct numbers, return all possible permutations.
For example,
[1,2,3] have the following permutations:
1. [
2. [1,2,3],
3. [1,3,2],
4. [2,1,3],
5. [2,3,1],
6. [3,1,2],
7. [3,2,1]
8. ]

Solution:
1. public class Solution {
2. public List<List> permute(int[] nums) {
3. List<List> res = new ArrayList<>();
4. 5. if (nums.length == 0)
6. return res;
7. 8. res.add(new ArrayList(Arrays.asList(nums[0])));
9. 10. for (int i = 1; i< nums.length; i++) {
11. List<List> next = new ArrayList<>();
12. 13. for (int j = 0; j <= i; j++) {
14. for (List list : res) {
15. // make a copy of existing perm
16. List copy = new ArrayList<>(list);
17. copy.add(j, nums[i]);
18. next.add(copy);
19. }
20. }
21. 22. res = next;
23. }
24. 25. return res;
26. }
27. }