leecode 回文字符串加强版

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

只考虑数字和字母，字母大小写都一样，判断是否是回文。重点是忽略非非字母和数字。比较简单。

 public class Solution {
     public boolean isPalindrome(String s) {

         int low=0;
         int high=s.length()-1;
         String s1=s.toLowerCase();
         while(low<high)
         {

             while(low<high)
             {
                char c=s1.charAt(low);
                if((c>='a'&&c<='z')||(c>='0'&&c<='9'))
                {
                    break;
                }
                else  low++;

             }

              while(low<high)
             {
                char c=s1.charAt(high);
                if((c>='a'&&c<='z')||(c>='0'&&c<='9'))
                {
                    break;
                }else high--;

             }

             if(s1.charAt(low)==s1.charAt(high))
             {
                 low++;
                 high--;
             }
             else
             {
                 return false;
             }

         }

         return true;

     }
 }