CODE

public class Solution {
  public List<List<int>> ThreeSum(int[] nums) {

    List<List<int>> result = new List<List<int>>();

    Array.Sort(nums);

    /* i - left marker, j - right marker, k - iterator */
    int i = 0;
    int j = nums.Length-1;
    int k = i + 1;

    /* proceed from left to right */
    while (i < nums.Length - 1)
    {
      /* for each i, check for triplet in remaining array */
      while (i < j)
      {   
        /* if sum of first two is neg, skip over remaining neg */
        if (nums[i] + nums[j] < 0)
          while (nums[k] < 0)
            k++;
        /* while iterator less than right marker, check for 3sum */
        while (k < j)
        {
          if (nums[i] + nums[j] + nums[k] == 0)
          {
            bool tripletAlreadyExists = false;
            List<int> t = new List<int>();
            t.Add(nums[i]);
            t.Add(nums[j]);
            t.Add(nums[k]);
            t.Sort();

            /* 3sum found, but check for dups */
            foreach (var triplet in result)
            {
              triplet.Sort();
              int count = 0;
              for (int a = 0; a < 3; a++)
              {
                if (triplet[a] == t[a])
                  count++;
              }
              if (count == 3)
              {
                tripletAlreadyExists = true;
                break;
              }
            }

            if (!tripletAlreadyExists)
            {
              result.Add(t);
              break;
            }
          }
          /* advance iterator */
          k++;
        }
        /* done with pass, move right marker left one, start over */
        j--;
        k = i + 1;
      }
      /* checked all from last left marker, move left marker right one, start over */
      i++;
      j = nums.Length - 1;
      k = i + 1;
    }
    return result;        
  }
}