Leetcode 125 - valid palindrome - second writing, through online judge, line 24 -27, else if logic has flaws - c1 not, c2 not, 3 cases, c1, or c2, or neither

Leetcode125-validPalindrome2.cs

public class Solution {
    public 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 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 char toUpper(char c)
        {
            int no = c - 'a';

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

highlights of practice 2:

1. Fail to pass the online judge
2. line 24 - 27, logic and reasoning has flaws
   both are ok
   both are failed
   A is failed
   B is failed
   Those are 4 cases - in the specific order as well.