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

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

if (nums == null || nums.length < 4) {

return res;

}

Arrays.sort(nums);

final int len = nums.length;

for (int i = 0; i <= len - 4; i++) {

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

continue;

}

for (int j = i + 1; j <= len - 3; j++) {

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

continue;

}

int start = j + 1, end = len - 1;

while (start < end) {

int sum = nums[i] + nums[j] + nums[start] + nums[end];

if (sum < target) {

start++;

} else if (sum > target) {

end--;

} else {

res.add(Arrays.asList(nums[i], nums[j], nums[start++], nums[end--]));

while (start < end && nums[start] == nums[start - 1]) {

start++;

}

while (start < end && nums[end] == nums[end + 1]) {

end--;

}

}

}

}

}

return res;

}