public static List<List<Integer>> fourSum(int[] nums, int target) {

List<List<Integer>> result = new ArrayList<>();

if (nums == null || nums.length < 4) return result;

Arrays.sort(nums);

int n = nums.length;

for (int i = 0; i < n - 3; i++) {

// Skip duplicates for i

if (i > 0 && nums[i] == nums[i - 1]) continue;

for (int j = i + 1; j < n - 2; j++) {

if (j > i + 1 && nums[j] == nums[j - 1]) continue;

int left = j + 1;

int right = n - 1;

while (left < right) {

long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];

if (sum == target) {

result.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));

while (left < right && nums[left] == nums[left + 1]) left++;

while (left < right && nums[right] == nums[right - 1]) right--;

left++;

right--;

} else if (sum < target) {

left++;

} else {

right--;

}

}

}

}

return result;

}

public static void main(String[] args) {

int[] nums = {1, 0, -1, 0, -2, 2};

int target = 0;

List<List<Integer>> result = fourSum(nums, target);

System.out.println("Quadruplets that sum to " + target + ": " + result);

// Example 2

int[] nums2 = {2, 2, 2, 2, 2};

target = 8;

System.out.println("Quadruplets that sum to " + target + ": " + fourSum(nums2, target));

}