Leetcode 125 - valid palindrome - 6th practice - follow the flow of real processing - skip left/right if need, comparison failed, or continue to compare

Leetcode125-validPalindrom6A.cs

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

namespace Leetcode125_IsPalindrome
{
    class Program
    {
        /*
         *  Leetcode 125: is palindrome 
         *  
         *  blog to read: 
         *  http://blog.csdn.net/nomasp/article/details/50623165
         */
        static void Main(string[] args)
        {
            bool test  = isPalindrome("Aa");
            bool test2 = isPalindrome("Ab%Ba");
            bool test3 = isPalindrome("B%1*1b"); 
        }
        
        

        /*
         * Function design:
         * 
         * 1. skip left if it is not alphabetic char or number -> a while loop 
         * 2. skip right if it is not alphabetic char or number -> a while loop
         * 3. if the left and right are nto equal, return false
         * 4. left pointer increments one, right pointer decrements one. 
         * 
         * The code is matching the above steps. 
         */
        public static bool isPalindrome(string s)
        {
            if (s == null || s.Length == 0)
                return true;

            int left = 0;
            int right = s.Length - 1;

            while (left < right)  
            {
                // skip left char
                while (left < right && !isAlnum(s[left]))                
                    left++;                 

                // skip right char
                while (left < right && !isAlnum(s[right]))
                    right--; 

                if (toUpper(s[left]) != toUpper(s[right]))
                    return false;

                left ++;
                right--; 
            }

            return true;
        }

        /*
         * a-z
         * A-Z
         * 0-9
         */
        private static bool isAlnum(char c)
        {
            const int SIZE = 26; 
            int[] arr = new int[] { c - 'a', c - 'A', c - '0' }; 
            
            // is A-Z or a-z or 0 -9
            if((arr[0] >= 0 && arr[0] < SIZE) || 
               (arr[1] >= 0 && arr[1] < SIZE) ||
               (arr[2] >= 0 && arr[2] <= 9 ) )
                return true;

            return false; 
        }

        /*
         * 
         */
        private static char toUpper(char c)
        {
            int no = c - 'a';

            if (no >= 0 && no < 26)
                return (char)('A' + no);
            else
                return c; 
        }
    }
}