Leetcode 125 - is palindrome - first writing

Leetcode125-isPalindrome1.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"); 
        }
        
        /*
         * August 19, 2016
         * 
         * define two functions: 
         * isAlnum()
         * toUpperCase()
         * 
         * highlights of first writing: 
         * 1. line 47 - work on positive case first, avoid too long discussion - let line 42 take care more things.
         * 2. static analysis: Runtime exception check, index-out-of-range
         * 3. line 51 - alternative: check Math.abs(c1-c2) = 26 or 0
         */
        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)
            {
                char c1 = s[left]; 
                char c2 = s[right];

                if (isAlnum(c1) && isAlnum(c2))
                {
                    if (toUpper(c1) == toUpper(c2))
                    {
                        left++;
                        right--;
                    }
                    else
                        return false;
                }
                else if (!isAlnum(c1))
                    left++;
                else
                    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; 
        }
    }
}

there is a bug in first writing, line 62, shoud be right--; not right++.
static analysis did not catch the bug - be careful next time.