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;
        
        
    }
}