3 sum leetcode

Solution.cs

using System.Collections.Generic;

public class Solution {
    public IList<IList<int>> ThreeSum(int[] nums) {
      var tripletsCollection = new List<IList<int>>(nums.Length);
      var tripletSet = new HashSet<string>(nums.Length);
      var numOccurance = new Dictionary<int, HashSet<int>>(nums.Length);
      
      for(int i = 0; i < nums.Length; i++)
        AddNumOccurance(numOccurance, nums[i], i);
      
      if(nums.Length < 3) return tripletsCollection;
      
      
      for(int i = 0; i < nums.Length; i++)
      {
        for(int j = i+1; j < nums.Length; j++)
        {
          var z = (nums[i]+nums[j]) * -1;
          if(numOccurance.ContainsKey(z))
          {
            var zSet = numOccurance[z];
            int cntr = 0;
            if(zSet.Contains(i)) cntr++;
            if(zSet.Contains(j)) cntr++;
                       
            if((zSet.Count - cntr) <= 0) continue;
            var possibleTriplet = new List<int>() { nums[i], nums[j],  z};
            possibleTriplet.Sort();
            var tripletHash = $"{possibleTriplet[0]}.{possibleTriplet[1]}.{possibleTriplet[2]}";
            if(tripletSet.Contains(tripletHash)) continue;
            tripletsCollection.Add(possibleTriplet);
            tripletSet.Add(tripletHash);
          }
            
        }
      }
      
      return tripletsCollection;
    }
  
  private void AddNumOccurance(Dictionary<int, HashSet<int>> dict, int num, int i)
  {
    if(!dict.ContainsKey(num))
      dict.Add(num, new HashSet<int>() { i });
    else dict[num].Add(i);
  }
  
}