<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)
  {
		if (list == null || list.Count == 0)
		{
			return null;
		}
		var lookup = new Dictionary<int int>();
		var array = list.ToArray();
		int length = array.Length;
		int count = 0;
		while (count &lt; length)
		{
			bool exists = lookup.ContainsKey(array[count]);
			switch (exists)
			{
				case true:
				lookup[array[count]]++;
				break;
				default:
				lookup.Add(array[count], 1);
				break;
			}
			count++;
		}
		int lowest = int.MaxValue;
		int key = 0;
		
		var lookupArray = lookup.ToArray();
		int lookupLength = lookupArray.Length;
		int currentLookupIndex = 0;
		while (currentLookupIndex &lt; lookupLength)
		{
			bool lower = lookupArray[currentLookupIndex].Value &lt; lowest;
			switch(lower)
			{
				case true:
					lowest = lookupArray[currentLookupIndex].Value;
					key = lookupArray[currentLookupIndex].Key;
					break;
				default:
					break;
			}
			currentLookupIndex++;
		}
		return key;
	}
  
}

</int></int></int></p>