0 <= a, b, c, d < n

a、b、c 和 d 互不相同

nums[a] + nums[b] + nums[c] + nums[d] == target

`class Solution {    public List<List<Integer>> fourSum(int[] nums, int target) {        List<List<Integer>> quadruplets = new ArrayList<List<Integer>>();        if (nums == null || nums.length < 4) {            return quadruplets;        }        Arrays.sort(nums);        int length = nums.length;        for (int i = 0; i < length - 3; i++) {            if (i > 0 && nums[i] == nums[i - 1]) {                continue;            }            if ((long) nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {                break;            }            if ((long) nums[i] + nums[length - 3] + nums[length - 2] + nums[length - 1] < target) {                continue;            }            for (int j = i + 1; j < length - 2; j++) {                if (j > i + 1 && nums[j] == nums[j - 1]) {                    continue;                }                if ((long) nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {                    break;                }                if ((long) nums[i] + nums[j] + nums[length - 2] + nums[length - 1] < target) {                    continue;                }                int left = j + 1, right = length - 1;                while (left < right) {                    long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];                    if (sum == target) {                        quadruplets.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));                        while (left < right && nums[left] == nums[left + 1]) {                            left++;                        }                        left++;                        while (left < right && nums[right] == nums[right - 1]) {                            right--;                        }                        right--;                    } else if (sum < target) {                        left++;                    } else {                        right--;                    }                }            }        }        return quadruplets;    }}`