PermutationsII.java

package backtracking;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * Created by gouthamvidyapradhan on 12/04/2017.
 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 PermutationsII
{
    public static void main(String[] args)
    {
        int[] A = {1, 2, 2};
        System.out.println(new PermutationsII().permuteUnique(A));
    }

    public List<List<Integer>> permuteUnique(int[] nums)
    {
        List<List<Integer>> result = new ArrayList<>();
        Arrays.sort(nums);
        nextPermutation(0, nums, result);
        return result;
    }

    private void nextPermutation(int i, int[] nums, List<List<Integer>> result)
    {
        if(i == nums.length - 1)
        {
            List<Integer> list = new ArrayList<>();
            for(int n : nums)
                list.add(n);
            result.add(list);
        }
        else
        {
            for(int j = i, l = nums.length; j < l; j++)
            {
                if(j > i && nums[j] == nums[i]) continue;
                swap(nums, i, j);
                nextPermutation(i + 1, Arrays.copyOf(nums, nums.length), result);
            }
        }
    }

    private void swap(int[] a, int i , int j){
        int tmp = a[i];
        a[i] = a[j];
        a[j] = tmp;
    }
}