<p>using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Xml;
using System.Text.RegularExpressions;

public interface ICodeRush
{
  int? FindLeastFrequentNumber(IList<int> list);
}

public class Solution : ICodeRush
{
  public int? FindLeastFrequentNumber(IList<int> list)
  {
            // return null for null or empty lists
            if (list == null || list.Count == 0)
            {
                return null;
            }

         Dictionary<int int> found = new Dictionary<int int>(list.Count / 4);
         for (int index = 0; index &lt; list.Count; ++index)
         {
            // Two hashes per update - could be worse I guess
            int current = list[index];
            int count;
            if (found.TryGetValue(current, out count))
            {
               ++count;
               found[current] = count;
            }
            else
            {
               found.Add(current, 1);
            }
         }

            int smallest = 0;
            int smallestVal = int.MaxValue;
    
            // Pay the penalty once to make this a list; faster to iterate after
            var data = found.ToList();
            for (int index = 0; index &lt; data.Count; ++index)
            {
               var item = data[index];
               if (item.Value &lt; smallestVal)
               {
                  // If we ever reach a 1, nothing will be lower, so just return this key now
                  if (item.Value == 1)
                  {
                     return item.Key;
                  }
                  // If this is smaller than the least frequent so far, store the key and value
                  else
                  {
                     smallest = item.Key;
                     smallestVal = item.Value;
               	  }
               }
            }
            
            return smallest;
  }
}

</int></int></int></int></p>