LeetCode18. 四数之和

给定数组 nums = [1, 0, -1, 0, -2, 2]，和 target = 0。

满足要求的四元组集合为：
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

public List<List<Integer>> fourSum(int[] nums, int target) {
            List<List<Integer>> ans = new ArrayList<>();
            Arrays.sort(nums);
            for (int i = 0; i < nums.length - 3; i++) {
                if (i > 0 && nums[i] == nums[i - 1]) {
                    continue;
                }
                for (int j = i + 1; j < nums.length - 2; j++) {
                    if (j > i + 1 && nums[j] == nums[j - 1]) {
                        continue;
                    }
                    int left = j + 1;
                    int right = nums.length - 1;
                    while (left < right) {
                        int sum = nums[i] + nums[j] + nums[left] + nums[right];
                        if (sum > target) {
                            right--;
                        } else if (sum < target) {
                            left++;
                        } else {
                            ArrayList<Integer> list = new ArrayList<>();
                            list.add(nums[i]);
                            list.add(nums[j]);
                            list.add(nums[left]);
                            list.add(nums[right]);
                            ans.add(list);
                            while (left < right && nums[left] == nums[++left]) {
                            }
                            while (left < right && nums[right] == nums[--right]) {
                            }
                        }
                    }
                }
            }
            return ans;
        }