[Java 101] LeetCode: 15. 3Sum with 2D List

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        Arrays.sort(nums);
        int N = nums.length;
        for (int i = 0; i < N - 2; i++) {
            if (i > 0 && nums[i] == nums[i-1]) {
                continue;
            }
            int j = i + 1, k = N - 1;
            while (j < k) {
                int n1 = nums[i], n2 = nums[j], n3 = nums[k];
                int sum = n1 + n2 + n3;
                if (sum == 0) {
                    List<Integer> temp = List.of(n1, n2, n3);
                    ret.add(temp);
                    while (j < k && nums[j] == n2) {
                        j += 1;
                    }
                    while (j < k && nums[k] == n3) {
                         k -= 1;
                    }
                } else if (sum < 0) {
                    j += 1;
                } else {
                    k -= 1;
                }
            }
        }
        return ret;
    }
}