Leetcode 3Sum problem solution in C#

public class Solution {
    public IList<IList<int>> ThreeSum(int[] nums)
        {
            Array.Sort(nums);
            var res = new List<IList<int>>();
            for (int i = 0; i < nums.Length && nums[i] <= 0; ++i)
            {
                if (i == 0 || nums[i - 1] != nums[i])
                {
                    TwoSum(nums, i, res);
                }
            }
            return res;
        }

        private void TwoSum(int[] nums, int i, List<IList<int>> res)
        {
            var a = i + 1;
            var b = nums.Length - 1;
            while (a < b)
            {
                var sum = nums[i] + nums[a] + nums[b];
                if (sum < 0 || (a > i + 1 && nums[a] == nums[a - 1]))
                    ++a;
                else if (sum > 0 || (b < nums.Length - 1 && nums[b] == nums[b + 1]))
                    --b;
                else
                {
                    var item = new List<int>();
                    item.Add(nums[i]);
                    item.Add(nums[a++]);
                    item.Add(nums[b--]);
                    res.Add(item);
                }
            }
        }
}