Game Throne - HackerRank string algorithm

GameThrone4.cs

using System;
using System.Linq;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.IO;
class Solution {
    static void Main(String[] args) {
        
        // input is a single line which will contain the input string
        var input = Console.ReadLine();
        
        // 1<=length of string <= 10^5
        if (input == null || input.Length < 1 || input.Length > 100000)
        {
            Console.WriteLine("NO");
            return;
        }
    
        // each character of the string is a lowercase english alphabet
        if (!Regex.IsMatch(input, "^[a-z]*$"))
        {
            Console.WriteLine("NO");
            return;	
        }
    
        // figure out how many times each character appears
        var charCount = new Dictionary<char, int>();
        foreach (var c in input)
        {
            if (!charCount.ContainsKey(c))
            {
                charCount[c] = 0;
            }
            charCount[c]++;
        }
    
        // palindrome exists if there is at most 1 character that appears an odd number of times
        if (charCount.Count(x => x.Value % 2 == 1) > 1)
        {
            Console.WriteLine("NO");
        }
        else
        {
            Console.WriteLine("YES");
        }
    }
}