<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; int b = list.Count; if (b &lt; 3) return list[0]; if (b &lt; 100) { int c = int.MaxValue, d = 0; for (int i = 0; i &lt; b; i++) { int e = 0, f = list[i]; for (int j = i; j &lt; b; j++) if (f == list[j]) e++; if (e == 1) return f; if (e &lt; c) { c = e; d = f; } } }
            var a = new Dictionary<int int>(); var g = new int[5000]; bool h = list == null, k = h; int l = g.Length, m = g[2666];
            for (int i = 0; i &lt; list.Count; i++) { int n = list[i]; if (n &gt; -1 &amp;&amp; n &lt; 5000){ g[n]++; k = true; if (l &gt; n) l = n; if (m &lt; n) m = n + 1; } else { if (a.ContainsKey(n)) a[n]++; else a.Add(n, 1); h = true; } }
            int o = int.MaxValue, p = 0; if (k) { for (int i = l; i &lt; m; i++) { if (g[i] &lt; o) { o = g[i]; if (o == 1) return i; p = i; } } } if (h) { foreach (var q in a) { if (q.Value &lt; o) { o = q.Value; if (o == 1) return q.Key; p = q.Key; } } }
            return p;
        }
}

</int></int></int></p>