Solution.java

public class Solution
{
    public List<List<Integer>> fourSum(int[] nums, int target)
    {
        Arrays.sort(nums);
        Set<List<Integer>> result = new HashSet<List<Integer>>();
        for (int i = 0; i < nums.length - 3; i++)
        {
            threeSum(i, nums, target, result);
        }
        return new ArrayList<>(result);
    }

    private void threeSum(int idx, int[] nums, int target, Set<List<Integer>> result)
    {
        for (int i = idx + 1; i < nums.length - 2; i++)
        {
            int j = i + 1, k = nums.length - 1;
            while (j < k)
            {
                int sum = nums[i] + nums[j] + nums[k] + nums[idx] - target;
                if (sum == 0)
                {
                    result.add(Arrays.asList(nums[idx], nums[i], nums[j], nums[k]));
                }
                j = sum < 0 || sum == 0 ? j + 1 : j;
                k = sum > 0 || sum == 0 ? k - 1 : k;
            }
        }
    }
}