Day-8 3Sum

public List<List<Integer>> threeSum(int[] nums) {
    Arrays.sort(nums);
    List<List<Integer>> res = new ArrayList<>();
    for (int i = 0; i < nums.length && nums[i] <= 0; ++i)
        if (i == 0 || nums[i - 1] != nums[i])
            twoSumII(nums, i, res);
    return res;
}
void twoSumII(int[] nums, int i, List<List<Integer>> res) {
    int lo = i + 1, hi = nums.length - 1;
    while (lo < hi) {
        int sum = nums[i] + nums[lo] + nums[hi];
        if (sum < 0 || (lo > i + 1 && nums[lo] == nums[lo - 1]))
            ++lo;
        else if (sum > 0 || (hi < nums.length - 1 && nums[hi] == nums[hi + 1]))
            --hi;
        else
            res.add(Arrays.asList(nums[i], nums[lo++], nums[hi--]));
    }
}

public List<List<Integer>> threeSum(int[] nums) {
    List<List<Integer>> res = new ArrayList<>();
    Set<Pair> found = new HashSet<>();
    Set<Integer> dups = new HashSet<>();
    Map<Integer, Integer> seen = new HashMap<>();
    for (int i = 0; i < nums.length; ++i)
        if (dups.add(nums[i]))
            for (int j = i + 1; j < nums.length; ++j) {
                int complement = -nums[i] - nums[j];
                if (seen.containsKey(complement) && seen.get(complement) == i) {
                    int v1 = Math.min(nums[i], Math.min(complement, nums[j]));
                    int v2 = Math.max(nums[i], Math.max(complement, nums[j]));
                    if (found.add(new Pair(v1, v2)))
                        res.add(Arrays.asList(nums[i], complement, nums[j]));
                }
                seen.put(nums[j], i);
            }
    return res;
}