CODE

public class Solution {
  List<List<int>> result = new List<List<int>>();
  public List<List<int>> Permute(int[] nums) {
    List<int> numsList = new List<int>();
    foreach (var n in nums)
      numsList.Add(n);

    permute(numsList);
    return result;        
  }

  List<int> perm = new List<int>();
  void permute(List<int> nums)
  {
    int r = nums.Count;
    for (int i = 0; i < r; i++)
    {
      int num = nums[i];
      perm.Add(num);
      nums.RemoveAt(i);

      if (nums.Count == 0)
      {
        List<int> permCopy = new List<int>(perm);
        result.Add(permCopy);
      }
      permute(nums);
      perm.Remove(num);
      nums.Insert(i, num);
    }
  }
}