验证回文字符串

给定一个字符串，验证它是否是回文串，只考虑字母和数字字符，可以忽略字母的大小写。

说明：本题中，我们将空字符串定义为有效的回文串。

示例 1:

输入: "A man, a plan, a canal: Panama"
输出: true

示例 2:

输入: "race a car"
输出: false

思路就是找到合适就比较不合适就往下一位继续找：

 public static bool Method1(string str)
        {
            char[] chars = str.ToLower().ToCharArray();
            int low = 0, high = str.Length - 1;
            while (low < high)
            {
                while (!(chars[low] <= 'z' && chars[low] >= 'a') && !(chars[low] <= '9' && chars[low] >= '0') && low < high)
                {
                    low++;
                }
                while (!(chars[high] <= 'z' && chars[high] >= 'a') && !(chars[high] <= '9' && chars[high] >= '0') && low < high)
                {
                    high--;
                }
                if (chars[low] != chars[high])
                {
                    return false;
                }
                low++;
                high--;
            }
            return true;
        }

      public static bool Method2(string str)
        {
            char[] chars = str.ToLower().ToCharArray();
            int low = 0, high = str.Length - 1;
            while (low < high)
            {
                if (!(chars[low] <= 'z' && chars[low] >= 'a') && !(chars[low] <= '9' && chars[low] >= '0') )
                {
                    low++;
                    continue;
                }
                if (!(chars[high] <= 'z' && chars[high] >= 'a') && !(chars[high] <= '9' && chars[high] >= '0') )
                {
                    high--;
                    continue;
                }
                if (chars[low] != chars[high])
                {
                    return false;
                }
                low++;
                high--;
            }
            return true;
        }