Permutations II

Permutations II

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:

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

public class Solution {
    public List<List<Integer>> permuteUnique(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return result;
        }
        
        Arrays.sort(nums);
        
        boolean[] visited = new boolean[nums.length];
        permuteHelper(nums, result, new ArrayList<>(), visited);
        
        return result;
    }
    
    void permuteHelper(int[] nums, List<List<Integer>> result, List<Integer> list, 
         boolean[] visited) {
        if (list.size() == nums.length) {
            result.add(new ArrayList(list));
            return;
        }
        
        for (int i = 0; i < nums.length; i++) {
            if (visited[i] || (i > 0 && nums[i - 1] == nums[i] && !visited[i - 1])) {
                continue;
            }
            
            list.add(nums[i]);
            visited[i] = true;
            
            permuteHelper(nums, result, list, visited);
            
            list.remove(list.size() - 1);
            visited[i] = false;
        }
    }
}

Hope this helps,
Michael