Permutations II

Given a list of numbers with duplicate number in it. Find all unique permutations.

Example:

For numbers [1,2,2] the unique permutations are:

[
  [1,2,2],
  [2,1,2],
  [2,2,1]
]

Code:

class Solution {
    /**
     * @param nums: A list of integers.
     * @return: A list of unique permutations.
     */
    public List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            res.add(new ArrayList<Integer>());
            return res;
        }
        Arrays.sort(nums);
        List<Integer> perm = new ArrayList<>();
        Set<Integer> visited = new HashSet<>();
        dfs(nums, perm, visited, res);
        return res;
    }

    private void dfs(int[] nums, List<Integer> perm, Set<Integer> visited,
                        List<List<Integer>> res) {
        if (perm.size() == nums.length) {
            res.add(new ArrayList<>(perm));
            return;
        }
        int prev = Integer.MIN_VALUE; // use to remove the duplicated solution in res
        for (int i = 0; i < nums.length; i++) {
            if (visited.contains(i) || prev == nums[i]) {
                continue;
            }
            perm.add(nums[i]);
            visited.add(i);
            dfs(nums,perm, visited, res);
            perm.remove(perm.size() - 1);
            visited.remove(i);
            prev = nums[i];
        }
    }
}