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.

答案

public class Solution {
    char toLowerCase(char c)
    {
        if(c>='A'&&c<='Z')
        {
            c+=32;
        }
        return c;
    }
    boolean isAlphanumeric(char c)
    {
        if(c>='a'&&c<='z')
        {
            return true;
        }
        if(c>='0'&&c<='9')
        {
            return true;
        }
        return false;
    }
    public boolean isPalindrome(String s) {
        if(s==null||s.length()<=1)
        {
            return true;
        }
        int i=0;
        int j=s.length()-1;
        char start;
        char end;
        while(i<j){
            start=toLowerCase(s.charAt(i));
            end=toLowerCase(s.charAt(j));
            if(!isAlphanumeric(start))
            {
                i++;
                continue;
            }
            if(!isAlphanumeric(end))
            {
                j--;
                continue;
            }
            if(start!=end)
            {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
}