Leetcode 125 - valid palindrome - code review - two pointers technique - filter once using O(n) time, and then check palindrome using O(n) time.

Leetcode125_validPalindrome_codeReview1.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Leetcode125_IsPalindrome
{
    class ValidPalindrome
    {
        /*
         *  Leetcode 125: Valid Palindrome
         *  https://leetcode.com/problems/valid-palindrome/?tab=Description
         */
        static void Main(string[] args)
        {
            RunTestcases();             
        }

        public static void RunTestcases()
        {
            Debug.Assert(IsPalindrome("Aa"));
            Debug.Assert(IsPalindrome("Ab%Ba"));
            Debug.Assert(IsPalindrome("B%1*1b"));
            Debug.Assert(!IsPalindrome("B%a*1b"));
        }

        /* 
         * Given a string, determine if it is a palindrom, considering
         * only alphanumeric characters and ignoring cases
         * Time complexity: 
         * O(N)
         *  
         * empty string is valid palindrome 
         */
        public static bool IsPalindrome(string rawString)
        {
            if (rawString == null || rawString.Length == 0)
            {
                return true;
            }

            string stringWithCases = fiterOutNonAlphanumbericaCharacters(rawString);

            // two pointers technique
            int start = 0;
            int end = stringWithCases.Length - 1;

            while (start < end)
            {
                char left  = toUpper(stringWithCases[start]);
                char right = toUpper(stringWithCases[end]);

                if (left - right != 0)
                {
                    return false;
                }

                start++;
                end--;
            }

            return true;
        }

        /*
         * check if the char is alphabetic or digit
         * a-z
         * A-Z
         * 0-9
         */
        private static bool isAlphabeticAndDigit(char anyChar)
        {
            if (isCapitalCaseAlphabetic(anyChar) ||
               isLowerCaseAlphabetic(anyChar)    ||
               isDigit(anyChar))
            {
                return true;
            }

            return false;
        }

        private static bool isCapitalCaseAlphabetic(char anyChar)
        {            
            var number = anyChar - 'A';
            return number >= 0 && number < 26;
        }

        private static bool isLowerCaseAlphabetic(char anyChar)
        {            
            var number = anyChar - 'a';
            return number >= 0 && number < 26;
        }

        private static bool isDigit(char anyChar)
        {            
            var number = anyChar - '0';
            return number >= 0 && number <= 9;
        }

        /*
         * assuming input char is alphabetic number, 
         * output the capitical case char
         */
        private static char toUpper(char alphabeticChar)
        {
            int number = alphabeticChar - 'a';

            if (number >= 0 && number < 26)
            {
                return (char)('A' + number);
            }            
            
            return alphabeticChar;            
        }
        
        /*
         * Filter out non alphanumeric characters
         * and keep cases
         */
        private static string fiterOutNonAlphanumbericaCharacters(string rawString)
        {
            return string.Concat( rawString.Where(c => isAlphabeticAndDigit(c) == true).ToArray());            
        }
    }
}