Given a collection of distinct 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],
[3,2,1]
]

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public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        permutation(result, new ArrayList<Integer>(), nums);
        return result;
    }
    
    public void permutation(List<List<Integer>> result, ArrayList<Integer> cur, int[] nums) {
        if (cur.size() == nums.length) {
            result.add(new ArrayList<Integer>(cur));
        } else {
            for (int i = 0; i < nums.length; i++) {
                if (cur.contains(nums[i])) continue;
                cur.add(nums[i]);
                permutation(result, cur, nums);
                cur.remove(cur.size() - 1);
            }
        }
    }
}